A motor cyclist participant of a race says "We drove with the…

A motor cyclist participant of a race says "We drove with the speed of 15 miles an hour one way, but while returning because of less traffic we drove on the same route with 21 miles per hour." What was their average speed in the whole journey?

This problem repeats another one in the library (at most with different numbers), so it's left out of browsing.

AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · train #41821