Question

Show a complete solution.
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?

Answer 1

let the speed of the current be x mph

so the time taken to go 9 miles upstream (against the current) = 9/(15-x)
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/(15-x)
cross-multiply
solve for x to get the speed (I got around 2.14)