If Judy rows at the rate of 7 mph against the current, and 11mph with the current, what is the average speed of her trip if she goes the same distance in both directions?
total distance / total time = average time = distance/rate
d = distance
d / 7 + d/11= 2d /average
1/7 + 1/11 = 2/average
average = 2 / (1/7 + 1/11) = 154/18 = 77/9 m/hr = 8.56 m/hr