A rower whose speed is 8 km/hr in still water rows to…

A rower whose speed is 8 km/hr in still water rows to a certain point upstream and back to the starting point in a river which flows at 4 km/hr. What is the rower's average speed (in km/hr) for the total 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 #9831