Mah equations

Question

A rowing team rowed 90 miles while going with the current in the same amount of time it took to row 10 miles against the current. The rate of the current was 4 miles per hour. Find the rate of the rowing team in still water.

Answer 1

So, if I row 3 mph but row with a 4 mph current, I'm going 7 mph. Rowing against the same current, I'm going backward 1 mph.

 

d = rt, so t = d/r, and the times are equal.

 

90/(r + 4) = 10/(r - 4)

 

Cross multiply!

 

90(r - 4) = 10(r + 4)

90r - 360 = 10r + 40

80r = 400

r = 5

 

They row at 5 mph.