Question 159561
Junior's boat will go 15 mph in still water. If he can go 12 miles downstream
 in the same amount of time it takes to go 9 miles upstream, then what is the
 speed of the current?
:
Let the speed of the current = x
then
(15+x) = speed downstream
and
(15-x) = speed upstream
:
The times are give as equal, write a time equation from this fact
remember; Time = {{{dist/speed}}}
:
Down stream time = Upstream time
{{{15/((15+x))}}} = {{{9/((15-x))}}}
Cross multiply, solve for x
9(15+x) = 15(15-x)
;
135 + 9x = 225 - 15x
:
9x + 15x = 225 - 135
:
24x = 90
x = {{{90/24}}}
x = 3.75 mph is the speed of the current
;
;
Check solution by finding the times of each trip (add & subtract the current)
15/18.75 = .8 hrs
9/11.25 = .8 hrs, confirms our solution