Math...
posted by Erin(im so confused!) on .
solve the porportion:
9/12=15/?
please help.

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 ? 
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