Question 1110579
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3 hours 36 minutes is 3 3/5 hours, or 18/5 hours.<br>
Let c be the rate of the current.  Then Bill's speed upstream is 3-c, and his speed downstream is 3+c.<br>
He is going 3 miles upstream and then 3 miles back; his total time is {{{3/(3-c)+3/(3+c)}}}.  So
{{{3/(3-c) + 3/(3+c) = 18/5}}}
{{{3(3+c)+3(3-c) = (18/5)(3-c)(3+c)}}}
{{{9+3c+9-3c = (18/5)(9-c^2)}}}
{{{18 = (18/5)(9-c^2)}}}
{{{5 = 9-c^2}}}
{{{c^2 = 4}}}
{{{c = 2}}}<br>
Answer: the speed of the current is 2 mph.