<PRE><B>Question 192008</B>
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.
:
Let c = speed of the current
:
(15+c) = speed downstream
and
(15-c) = speed upstream
:
Write a time equation; Time = {{{distance/speed}}}
Time down = time up
{{{12/((15+c))}}} = {{{9/((15-c))}}}
Cross multiply
9(15+c) = 12(15-c)
135 + 9c = 180 - 12c
9c + 12c = 180 - 135
21c = 45
c = 2.143 mph is the speed of the current
:
:
Check solution by finding the times using a calc:
{{{12/((15+2.143))}}} = 7.0
{{{9/((15-2.143))}}} = 7.0 hrs

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