Here is a problem from way back from my bro's math (elementary)
3 cats eat 3 cats in 3 minutes
how long does it take for 100 cats to eat 100 fish?
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(hint: answer is not 100 minutes, and if you answer 100 minutes you are wrong and will be reported)

3 cats 3  fish 3 minutes
100 cats100 fish  x

The cats and fish are variables directly proportional, the cats and the time are varibles inversely proportional,
Therefore, 

3 cats * 100 fish * x=100 cats * 3 fish * 3 minutes
x=(100 cats * 3 fish * 3 minutes) / (3 cats * 100 fish)
x=3 minutes.

the time will be 3 minutes.