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# Worked example: Rate problem

CCSS.Math:

## Video transcript

a squirrel is running across the road at 12 feet per second it needs to run nine feet to get across the road how long will it take the squirrel to run nine feet around your round to the nearest hundredth of a second fair enough a car is 50 feet away from the squirrel okay they said this is a high stakes word problem driving toward it at a speed of 100 feet per second how long will it take the car to drive 50 feet round to the nearest hundredth of a second will the square will make it nine feet across the road before the car gets there so this definitely is high stakes at least for the squirrel so let's answer the first question let's figure out how long will it take the squirrel to run nine feet so let's think about it the squirrel we write squirrel squirrel right over here the squirrel so the squares got to go nine nine feet and we have we want to figure out how many seconds it's going to take so what would we would we divide or multiply this by twelve well to think about that you could think about the units where we want to get an answer in terms of seconds we want to figure out time so it'd be great if we could multiply this times seconds seconds per seconds per foot then the feet will cancel out and I'll be left with seconds now right over here we're told that the squirrel can run a 12 feet per second but we want seconds per foot so the squirrel every second so they go 12 feet per second we then we could also say one second per every 12 feet so let's write it that way so it's essentially the reciprocal of this because the unit's are the reciprocal of this so it's one second for every 12 12 feet notice all I did is I took this information right over here 12 feet per second and I wrote it a second per foot 12 feet for every one second one second for every 12 feet what's useful about this is this will now give me the time it takes for the squirrel in seconds so the feet cancel out with the feet and I am left with nine times 1/12 which 912 seconds and 912 seconds is the same thing as 3/4 seconds which is the same thing as 0.75 seconds for the squirrel to cross the street now let's think about the car so now let's think about the car and it's the exact same logic they tell us that the car is 50 feet away so it's the squirrels trying to do the squirrel is trying to cross the road like that and the car is 50 feet away coming in like that and we want to figure out if the squirrel will survive so the car is 50 feet away so it's 50 feet away we want to figure out the time it'll take to travel at 50 feet once again we would want it in seconds so we would want seconds per feet so we would want to multiply by seconds per foot seconds per foot they give us the speed in feet per second 100 feet per second and so we just have to realize that this is 100 feet for every one second or 1/100 seconds per feet this is once again just to this information this information but we took the reciprocal of it because we don't want feet per second we want seconds per feet and if we do that that cancels with that and we're left with 50 over 100 seconds so this is 50 over 100 is 0.5 o seconds and so now let's answer the question this life-and-death situation for the squirrel will the squirrel make it 9 feet across the road before the car gets there we also going to take the squirrel 0.75 seconds to cross that's going to take the car only half a second so the car is going to get to where the squirrel is crossing before the squirrel has a chance to get all the way across the road so unfortunately for the squirrel the answer is No