A professional rower can paddle 320 miles upstream in 10 hours and downstream 320 miles in 5 hours. How fast is the current? (don't forget your units)
Let R = rate of the rower in still water.
Let S = rate of the stream.
Downstream, the final rate is R + S.
Upstream, the final rate is R - S.
Using the formula Distance = Rate x Time:
Downstream: 320 = (R + S)(5) ---> 320 = 5R + 5S ---> x2 ---> 640 = 10R + 10S
Upstream: 320 = (R - S)(10) ---> 320 = 10R - 10S
Combining the equations: 640 = 10R + 10S
320 = 10R - 10S
Adding down the columns: 960 = 20R ---> R = 960 / 20 = 48 mph
To Find S: 320 = 5R + 5S ---> 320 = 5(48) + 5S ---> 320 = 240 + 5S ---> 80 = 5S ---> S = 16 mph