Sam is doing his math homework. hey completes 10 probelms every 30 minutes. how many porbelms can sam complete in 60 minutes?

Put the ratios into fraction form. Since you do not know how many problems, replace it with an unknown number a.k.a. a variable. 

10/30 = x/60

Since you can get 30 to 60 by multiplying by 2, multiply everything on the left side by 2.

Then, you get 20/60, which means 20 = x. So, he can complete 20 minutes an hour.

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