P.S. I skipped steps and separated things out. The original equation would be 8/12 = X/21 (which is basic algebra). Then it becomes 21*(8/12) (which is multiplying fractions).

Since I know how to multiply fractions, I can take the 8 out momentarily and look at 21/12 = 1 and 9/12 = 1 and 3/4. Then I multiply that by 8 so I know I have 8 + 8*(3/4) = 8 + 6 = 14.