What you are trying to do is isolate the variable x. For you to do that you have to first simplify the left side of the equation and then you solve for x.
3/45 = 20/x...3 in to 45 seconds = 20 in to x seconds cross multiply (3)(x) = (45)(20) 3x = 900 x = 900/3 x = 300...300 seconds (or 5 minutes)to fill the tub with 20 inches
