Solve for x. 12:15 = x:5

15/3 = 5

12/3 = 4

x = 4

You will have to isolate x by multiplying both sides by 5. It'll leave you with (12x5) / 15. Then you just do parentheses first, 12x5=60. After you just divide 60 by 15, 60/15=4.