Question

Cities A, B, C and D are equidistant. That is, B is d miles from A, C is d miles from B, and D is d miles from C. Suppose you travel 55 mph from A to B, 40 mph from B to C, and 50 Mph from C to D. What single, constant speed could you have traveled from A to B to C to D that would have taken the same amount of time?
Robo
I know that I am supposed to use harmonic mean. Just not sure how to set up formula.