Question

solve the porportion:

9/12=15/?

please help.

Answer 1

9 and 15 are the numerators
12 and ? are the denominators

Start the problem by setting up an equation by multiplying the numerators with the denominators like this:

9 x ? = 12 x 15

Now just solve for ?

Answer 2

It is ok to cross multiply but I prefer to multiply both sides by the product of the denominators. It is easier to justify by the rules of equation solving.
(9/12)(12 x) = (15/x)(12 x)
9 x = 15*12