Posted by Sandra on .
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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?

algebra 
Reiny,
let the speed of the current be x mph
so the time taken to go 9 miles upstream (against the current) = 9/(15x)
and the time taken to go 12 miles downstream (with the current) = 12/(15+x)
but we are told the times are the same, so
12/(15+x) = 9/(15x)
crossmultiply
solve for x to get the speed (I got around 2.14)