how do you get the answer to 8/12+?=1

Question

8/12 + (  ) =1

Answer 1

Think it through.  You're start with 8/12 or 8 twelfths.  1 is the same as 12/12 or 12 twelfths.  So how many twlefths do you need to add to 8 of them to get 12 of them.  The answer is 4.

 

So the number in parentheses is 4/12.