Question

Write the missing numerator for 15/25=x/5

Answer 1

25 was divided by 5 to get 5
so you have to divide 15 by 5 to get the numerator.
15/5 = 3 

the numerator is 3.

Answer 2

15/25 would equal 3/5, since the common multiple would be 5. By dividing by 5 for 15 and 25 would equal 3 and 5. Put it into a fraction, and you will get 3/5. 
Hope this helps!