Question

it takes billy fifteen minutes to complete 1/8 of a recipe. at this rate how long will it take for him to complete the recipe?

Answer 1

For this question, your equation would be 
15*8=120 because fifteen would represent that one part of 8 in the recipe. 

It would take 120 minutes or two house to complete the recipe at that rate

Answer 2

15 times 8 = 120 because he completed 1/8th in 15 minutes.
120 divided by 60, which is an hour, is 2. 
Therefore, it took Billy 2 hours to complete his recipe.