You are travelling to point B from point A at a speed of 30mph. You travel back to point A from Point B (taking the same route) at 60mph. What is your average speed?

(In reply to Here's the solution by suresh)

When the distances are not necessarily equal, the average speed is still a harmonic mean, but one with unequal weights.  The average will be the weighted harmonic mean

1/((w_1/s_1)+(w_2/s_2)+...+(w_n/s_n))

where the s_i is the speed over the distance d_i, and w_i is d_i over the sum of all the distances (so the w_i sum to unity).

Posted by Richard on 2006-10-31 11:58:54