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Measurement word problem: running laps

Sal solves a US customary distance word problem involving miles, yards, and feet. Created by Sal Khan and Monterey Institute for Technology and Education.

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  • leafers ultimate style avatar for user Thibaut
    Why isn't there a video on the eternal question: "How many fluid ounces will fit into X amount of cups?" because, I'm sorry, I can't figure it out. I've taken the time to understand it for HOURS now and I can't figure it out. It gets to the point where I start to ask questions that are irrelevant to the math, always a great sign: "Why do I need to figure how many fluid ounces fit into how many cups? Why is this relevant? When will I EVER need this skill?" I take the time to carefully read the instructions and the question (and of course, the hints afterwards) with - over the years over trying over and over - a figurative team of scientists behind me and I still can't figure this out. Everytime I think I understand the workings of this small exam created by aliens, the answer I give in is wrong, and it's yet again back to the drawing board, over and over and over AND OVER again.

    I'm sorry, but this is getting to the point of stupidity. Or it already got to that point. Can somebody, in plain and simple language, explain to me how much cranberry juice Molly needs? Because, judging by the explanation and this entire concept, I seriously don't think she knows.
    (2 votes)
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    • duskpin seedling style avatar for user Dogeleitom1
      Good question. Let's say Sailor has 32 pints of lemon juice, but Scarlett tried to figure out how many gallons of lemon juice Sailor had to make it easier. Well, we know that 16 pints is a gallon, so technically, 16x2 equals 32. Sailor really has 2 gallons.
      (4 votes)
  • hopper happy style avatar for user HaydenG
    do not understand
    (6 votes)
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  • blobby green style avatar for user braydendegrace
    how do you convert square meters back to meters
    (2 votes)
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  • leaf green style avatar for user lana young
    D runs circular track in 120 seconds. B running in opposite direction meets d every 48 seconds. S running in same direction as B, passes B every 240 seconds. How often does S meet D?
    (2 votes)
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    • duskpin ultimate style avatar for user William
      While this may seem like a complex problem at first glance, you can take a very simple and logical shortcut. First, recognize that the track length is insignificant and has no effect on the problem. Units are also irrelevant, except noting that your final answer will be in seconds. Then find that you can change the speed of D, while having no effect on S and B, since they just change proportionally in response.

      Thus, assume the length of the track is 1 and D is stationary. (Their speeds in this special scenario represent their relative speeds in the problem we are actually doing, which can be used to solve for when they meet. From now on, all "speeds" I mention in this scenario will refer to their relative actual speeds). Obviously, that would mean B runs at a speed of 1/48 using distance = rate x time. Then, for S to meet B in 240 seconds, S would have to run an additional 1 length (one more lap of length 1). That means that in 240 seconds, the distance traveled by S, which is the rate (r) times 240, is one more than the distance traveled by B, which is 1/48 times 240, since you already solved the speed of B. You can write this as the equation 240r = 240/48 + 1. Solving for r, we get 1/40 as the speed of S. Since we assumed D to be stationary, the time it takes for S to meet D is just the time it takes for S to make a full lap, or 40 seconds, using d=rt once more.

      I hope this helps, if anything is confusing please let me know so I can clarify! :)
      (2 votes)
  • blobby green style avatar for user jlmhudson.910
    where did you get the 1/3 at
    (2 votes)
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    • stelly blue style avatar for user Kim Seidel
      1 yard equals 3 feets. So, the conversion ratio from feet to yards is 1 yard / 3 feet. I believe this is the 1/3 that you are asking about.

      If you measure 12 feet and want that converted to yards, you would multiply 12 feet x 1 yard / 3 feet = 4 yards.
      (3 votes)
  • blobby green style avatar for user Donnailla Johnson
    Jeff takes 8 minutes to walk a lap. Susan taste 6 minutes to walk a lap. At what point would they walk the same amount of laps?
    (2 votes)
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  • female robot grace style avatar for user Moksha
    At . What is the purpose of doing things like ft/mile when you can just make it easier by dividing the 5,280 feet by 3? (because there are 3 ft in a yard)
    (3 votes)
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    • leaf yellow style avatar for user Madialyn Neyohaven
      I honestly agree with you(some people find this method easier). You totally could find the number of feet and divide it by three, instead of going through the process of multiplying it by 1/3(with all the extra ft, mi, and yd stuff). I think Sal did it this way so that people understand what units they're working with in each step. It's really easy to get lost with the units in these types of problems.
      (0 votes)
  • aqualine ultimate style avatar for user vlacsamouram88
    how many inches are in a foot
    (2 votes)
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  • leaf red style avatar for user TheAttack
    While I'm clear with most on the material on the exercises for this part of the unit, I've been having a tough time conceptualizing the "wall" questions. You don't exactly solve them using dimensional analysis (at least when I saw how KA solved it. Please correct me if I'm wrong), so it messes with my brain. For example, a question would go as the following:

    "It takes 36 minutes for 7 people to paint 4 walls...How many minutes does it take 9 people to paint 7 seven walls?"

    So my mind takes some really messy pathways in an attempt to solve the problem, leading to never ending loop of trial and error. Therefore, could anyone breakdown the process of solving the problem so I could understand how it is solved and why you would solve it that way? Thanks.
    (2 votes)
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    • piceratops tree style avatar for user VVCephei
      Each person paints walls with a certain speed measured in (w)alls/(m)inute. Let's say it is xw/ym. And since there are 7 people, we can assume that the overall speed with which walls are getting painted is 7 times that, or 7xw/ym, And we know that the overall speed was 4 walls/36 minutes, or 1w/9m. Now we can set up an equation: 7xw/ym = 1w/9m. And to figure our what xw/yw equals to, we just need to multiply both parts by 1/7. 7xw/ym * 1/7 = 1w/9m * 1/7 = xw/ym = 1w/63m. So it takes 63 minutes for 1 person to paint a wall.

      Now that you know the speed, you can set up the second equation: 9 * 1w/63m = 7w/xm = 9w/63m = 7w/xm = 1w/7m = 7w/xm *1/7 = 1w/7m * 1/7 = 7w/xm * 1/7 = 1w/49m = 1w/xm. So, x = 49.
      (2 votes)
  • primosaur ultimate style avatar for user empty
    why is the man in the video wants to covert mile to feet instead not just yard?
    (2 votes)
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Video transcript

Jamir is training for a race and is running laps around a field. If the distance around the field is 300 yards, how many complete laps would he need to do to run at least 2 miles? So they tell us how far one lap is, it's 300 yards, but we need to figure out how many laps to go 2 miles. So a good starting point would be to get everything into the same units. We have distance here in terms of miles, we have it here in terms of yards. So let's just get everything into yards. So he needs to run 2 miles. How do we convert that to yards? Well, I don't have it memorized how many yards there are per mile, but I do have it memorized how many feet there are per mile. And it's a good thing to have in the back of your brain someplace, that in general you have 5,280 feet per mile. It's a good number to know. 5,280 feet per mile. So if we want to convert, we can first convert the miles to feet, and then we know that there are 3 feet per yard, and then we'll have 2 miles in terms of yards. So 2 miles, if we want it converted to feet, we want miles in the denominator and we want feet in the numerator. And the reason why I say that is so that this miles will cancel out with that miles, and we'll just have feet there. And I just wrote down, there's 5,280 feet per mile, or you say 5,280 feet for every 1 mile. You can write it either way, but let's just write it like that. And then we can multiply. So this is going to give us what? If we just multiply the numbers 2 times 5,280. So what is that going to be? Maybe I should get a calculator out. Or we could do that in our head. Let's think of it this way: 2 times 80 is 160. 2 times 200 is 400. So it's going to be 400 plus 160 is going to be 560. And then 2 times 5,000 thousand is 10,000. So it's 10,560. And then the miles cancel out, and we are just left with feet. And let me actually multiply it out. I did it in my head that time, but that's not always useful. Let me verify for you that 5,280 times 2 is indeed 10,560. So 2 times 0 is 0. 2 times 8 is 16. Carry the 1. 2 times 2 is 4, plus 1 is 5. 2 times 5 is 10. 10,560. So he needs to run 10,560 feet. Now, we want this in terms of yards. So 10,560 feet. Let's convert this to yards. Well, we want it in yards. So we want yards in the numerator, and we want feet in the denominator, so that the feet cancel out with that feet right there. And we know that there are 3 feet for every 1 yard. Or another way to read this is that you have 1/3 of a yard for every foot. And now we can multiply. And it makes sense. If you're going from feet to yards, the number should get smaller because yards is a bigger unit. You need fewer yards to go the same distance as a certain number of feet. So it makes sense that we're dividing. Same thing: 2 miles is a ton of feet, so it made sense that we were multiplying by a large number. Here it makes sense that we're dividing. So let's do this. So this becomes 10,560 times 1 divided by 3. So it's 10,560/3. That's that and that part. And then the feet cancel out, and we are just left with yards. So 2 miles is 10,560 divided by 3. And let's figure out what that is. So 3 goes into 10,560. It doesn't go into 1. It goes into 10 three times. 3 times 3 is 9. And we subtract. We get 1. Bring down this 5. It becomes a 15. 3 goes into 15 five times. 5 times 3 is 15. We have no remainder, or 0. You bring down the 6. 3 goes into 6 two times. Let me scroll down a little bit. 2 times 3 is 6. Subtract. No remainder. Bring down this last 0. 3 goes into 0 zero times. 0 times 3 is 0. And we have no remainder. So 2 miles is the equivalent to 3,520 yards. That's the total distance he has to travel. That's the equivalent of 2 miles. Now we want to figure out how many laps there are. We want this in terms of laps, not in terms of yards. So we want the yards to cancel out. And we want laps in the numerator, right? Because when you multiply, the yards will cancel out, and we'll just be left with laps. Now, how many laps are there per yard or yards per lap? Well, they say the distance around the field is 300 yards. So we have 300 yards for every 1 lap. So now, multiply this right here. The yards will cancel out, and we will get 3,520. Let me do that in a different color. We will get 3,520, that right there, times 1/300. When you multiply it times 1, it just becomes 3,520 divided by 300. And in terms of the units, the yards canceled out. We're just left with the laps. So this is how many laps he needs to run. So 3,520 divided by 300. Well, we can eyeball this right here. What is 11 times 300? Let's just approximate this right here. So if we did 11 times 300, what is that going to be equal to? Well, 11 times 3 is 33, and then we have two zeroes here. So this will be 3,300. So it's a little bit smaller than that. If we have 12 times 300, what is that going to be? 12 times 3 is 36, and then we have these two zeroes, so it's equal to 3,600. So this is going to be 11 point something. It's larger than 11, right? 3,520 is larger than 3,300. So when you divide by 300 you're going to get something larger than 11. But this number right here is smaller than 3,600 so when you divide it by 300, you're going to get something a little bit smaller than 12. So the exact number of laps is going to be a little bit lower than 12 laps. So 2 miles is a little bit lower than 12 laps. But let's make sure we're answering their question. How many complete laps would he need to do to run at least 2 miles? So they're telling us that, look, this might be, 11 point something, something, something laps. That would be the exact number of laps to run 2 miles. But they say how many complete laps does he have to run? 11 complete laps would not be enough. He would have to run 12. So our answer here is 12 complete laps. That complete tells us that they want a whole number of laps. We can't just divide this. If we divide this, we're going to get some 11 point something, something. You can do with the calculator or do it by hand if you're interested. But we have to do at least 12 because that's the smallest whole number of laps that will get us to at least this distance right here, or this number of laps, or the equivalent of 2 miles.