SOLUTION: PLEASE, HELP ME WITH THIS PROBLEM! I'VE BEEN TRYING SINCE YESTERDAY AND I COULDN'T FINISH YET =( THANKS. In 8h, two campers rowed 15min down a river and then rowed back their camp

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Question 170608: PLEASE, HELP ME WITH THIS PROBLEM! I'VE BEEN TRYING SINCE YESTERDAY AND I COULDN'T FINISH YET =( THANKS.
In 8h, two campers rowed 15min down a river and then rowed back their campsite. The rate of the river's current was 1mph. Find the rate at which that campers row in calm water.
Answer by scott8148(5891) About Me  (Show Source):
You can put this solution on YOUR website!
assuming min is mi. __ t=d/r __ let x=rate in calm water

15/(r+1) + 15/(r-1) = 8 __ multiplying by (r+1)(r-1) __ 15r-15 + 15r+15 = 8r^2-8

8r^2-30r-8=0 __ dividing by 2 __ 4r^2-15r-4=0

factoring __ (4r+1)(r-4)=0

4r+1=0 __ r=-1/4 __ negative value not realistic

r-4=0 __ r=4 mph