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# Writing proportions example

Some examples of writing two ratios and setting them equal to each other to solve proportion word problems. Created by Sal Khan.

## Want to join the conversation?

#1: \$5.78
#2: 11.2 apples
#3: 6 eggs

I'm only asking because I'm not sure if I fully understand this unless I get the answer right...
• and im pretty sure you wouldnt add 11.2 apples it would be 11.
• Lonnie was paid 115.50 for 14 hours of work. work At this rate,how much will Lonnie earn for working 22 hours? How do I set up the problem.
• (115.5/14)*22= \$181.5
(sorry I just do it a very different way.)
• This equations make no sense, I have no idea on how to learn them, for example; here i have one;

5 erasers cost \$9.85.
Which equation would help determine the cost of 9 erasers?
Normaly I will divide the price by 5 to determin the cost of one eraser and then multiply by 9 to determin the cost of 9, but here the system gives me this;

5/9=\$9.85/x as a final equation that I should have done it ?
Can someone explain to me how this equation makes sense?
• Same trouble here. I don't understand why the cost is the denominator.
• I got 8.96 for question one, 9 divided by 11.50 gives me 1.277 ( repeated sevens) rounded off gives 1.28, times 7 gave me 8.96. Is this incorrect?
• 9 divided by 11.5 does not = 1.2777...
Sounds like you actually did 11.5/9 which does = 1.2777..., which is ok.
Multiply that by 7 and you get the actual answer of 8.94
Your answer is a little off because you rounded too early. Keep 3-4 of the 7's rather than rounding when you did. 1.27777 * 7 = 8.94439, then round to 8.94
• I wonder why he doesn't solve it
• Because it's not in the lesson to solve it, at the very beginning, he even says, "What I want to do in this video is not solve the word problems, but just solve the equation."
(1 vote)
• For the last one cant you just do 2 times 3
• For the ratios of the first problem, if you're confused to how he got 11.50/9 = x/7.
Well, we know 9 markers cost \$11.50. Let's say we do an equation with a constant to find out how to get how much a certain number of markers cost.

So: \$11.50 = k * 9. That k represents the cost of 1 marker. Makes sense, right? Because the cost of 1 marker times 9 markers is \$11.50.
Now, you can solve for k by seeing that: \$11.50/9 * 9 = \$11.50.
So: k = 11.50/9.
Now, do the same for the seven markers. x = cost of 7 markers.
x = k*7. Solve for k? k = x/7. Note the k is constant and is equal to the k for nine markers. So if k = k, we can say 11.50/9 = x/7.
Now for 9/11.50 = 7/x, you just flip the variables around in the equations we wrote and it changes the meaning of k.
Btw, if you divide that 11.50/9 you get roughly \$1.28 per marker or \$1.277777 repeating forever for every marker. Since the k is the same ratio, you can plug that in for the equation of x, and multiply \$1.28 x 7 to get \$8.98, which is close to the cost of 7 markers: not quite since it's an estimate but it is good enough.
If you want to get the actual value you would multiply that 1.2777777778 by 7 to get 8.944444, which you can round to \$8.94. I prefer just using \$1.28 x 7 since it's only like 4 pennies off and it's easier to calculate.
• i am very confused can you please make a video and simplify it