y:15=7:3

So we have

(y/15) = (7/3)

Using cross-product equality, we have

3y = 15*7

3y = 105 

divide by 3

y = 35

Here's another method that sometimes works well. What did I have to multiply 3 by in the denominator of the second fraction to get 15 in the denominator of the other??  Answer, 5.  So, if I multiply the 7 in the numerator of the second fraction by 5, I get 35 for "y" in the numerator of the first fraction.

A simple ratio/proportion problem.

y:15 = 7:3

y/15 = 7/3

y/15 = 35/15

y = 35