Question

Abby walks from town A to town B at a constant speed of 3mph, and back from town B to town A at a constant speed of 5mph.
A. what is Abby's average speed on the round trip walk (it is not 4mph).
B. what is Abby's average speed on the round trip if she walked one way at "a" mph and returned at "b" mph?

Answer 1

(3-5)/2
-2/2
-1

Answer 2

how can it be negative?

Answer 3

a) If you treat AB and BA as positive vectors then the average is 4 mph. However, if you assign AB as the positive vector and BA as negative vector (since one direction is too town and the other is from town), then you get a -1 mph, you could reverse the signs and come up with 1 mph, per imranmeah91's calcs.

b) (a+b)/2, where 'a' and 'b' are given appropriate signs regarding directions (+/-).