If one walks 500 feet at 4 miles per hour walking to hisher

If one walks 500 feet at 4 miles per hour walking to his/her destination and walks back 500 feet at 3 miles per hour... What is the average speed for the entire trip? How do you calculate it?

Solution

d = rt, where d=distance, r=rate, t=time
d = 500 ft
r = 4 miles per hr (going one way)
r = 3 miles per hr (going the other way)
.
We need to get everything into feet:
4 miles per hr = 4*5280 ft per mile = 21,120 ft per hr
3 miles per hr = 3*5280 ft per mile = 15,840 ft per hr
.
At 4 mph:
500 = 21120 t
t = .02367 hr
.
At 3 mph:
500 = 15840 t
t = .03156 hr
.
The total distance walked: 1000 feet
The total time: .02367 + .03156 = .05523 hrs
1000 = .05523 r
r = 18,106 feet/hr
dividing by 5,280 feet/mile we have
r = 4.329 miles per hr, which is the average rate