algebra
posted by Linda on .
36 Upstream, downstream. Junior’s boat will go 15 miles per hour in still water. If he can go 12 miles downstream in the same amount of time as it takes to go 9 miles upstream, then what is the speed of the current?

I know how they got 21 by adding 9 and tewnty ine but I don't know where the 45 camefrom,I know 9/15 + 12/15, but how did the 45 get in there?