Question

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?

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