Answer:
To cross multiply, multiply the numerator from one side with the denominator of the other side and put that product on one side of the equation. Do the same with the other numerator/denominator and put that on the other side. From there, solve for the unknown part.
So your ratio might look like this:
4/6=x/12
6x=12(4)
6x=48
x=8
So in this case 4/6 would equal 8/12.
HTH :)