<PRE><B>Question 79960</B>
In still water, a boat averages 40 miles per hour. It takes the same amount of time to travel 85 miles downstream, with the current, as 75 miles upstream, against the current. What is the rate of the water’s current?
:
Let x = current rate
:
(40+x) = speed downstream
(40-x) = speed upstream
:
Write a time equation; time = distance/speed
:
{{{85/(40+x)}}} = {{{75/(40-x)}}}
:
Cross multiply:
75(40+x) = 85(40-x)
:
3000 + 75x = 3400 - 85x
:
75x + 85x = 3400 - 3000
:
160x = 400
:
x = 400/160
:
x = 2.5 mph is the current
:
:
Check by finding the distance at each speed:
85/42.5 = 2 hrs
75/37.5 = 2 hrs also



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