Here's what i did:
I broke down the fraction entirely.
3 / 5 = 6 / 10
divide 6 by two and divide 18 by two. This makes 3 / 10 = 9.
divide 3 by three and 9 by three, making 1 / 10 = 3.
if you need 7 / 10 of the number (30) then do 3*7.
The answer is 21.