Question 360699
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Hi,
*Note: d = r*t or t = d/r
IT takes the same amount of time ...
Let r represent speed in still water
{{{(15/(r + 5mph)) = (9/(r - 5mph))}}}
cross multiply and solve for r 
15(r-5)= 9(r+5)
15r - 75 = 9r + 45
6r = 120
r = 20 mph
checking our answer
15/25 = 9 /15 = 3/5