A car travels along a straight line at a constant speed of 53.5 mi/h for a distance d and then another distance d in the same direction at another constant speed. The average velocity for the entire trip is 34.0 mi/h.
(a) What is the constant speed with which the car moved during the second distance d?
please explain this too how 2d/(d/v1 +d/v2) = 2v1v2/v1+v2