Question 1159172
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If r is the speed in mph of the boat in still water, then its upstream speed is r-5 and its downstream speed is r+5.<br>
upstream time: 20/(r-5)
downstream time: 20/(r+5)<br>
The upstream time is 8 1/3 hours (25/3 hours) greater than the downstream time:<br>
{{{20/(r-5) = 20/(r+5)+25/3}}}
Multiply everything by the least common denominator, 3(r-5)(r+5):<br>
{{{60(r+5) = 60(r-5)+25(r^2-25)}}}
{{{60r+300 = 60r-300+25r^2-625}}}
{{{1225 = 25r^2}}}
{{{r^2 = 49}}}
{{{r = 7}}}  (clearly the r = -7 solution makes no sense...)<br>
ANSWER: The speed of the boat in still water is 7mph<br>
CHECK:
upstream speed: 20/(7-5) = 20/2 = 10
downstream speed: 20/(7+5) = 20/12 = 5/3 = 1 2/3
10 - 1 2/3 = 8 1/3<br>