Problem: a rower whose speed is 4 km / hr in still water rows to a certain point upstream and back to the starting point in a river which flows at 2 km / hr . what is the rower ' s average speed ( in km / hr ) for the total journey ?
Rationale: "time upstream = d / 2 time downstream = d / 6 total time = d / 2 + d / 6 = 2 d / 3 average speed = 2 d / ( 2 d / 3 ) = 3 km / hr the answer is d ."
annotated_formula: divide(add(subtract(4, const_0.5), add(4, 2)), const_2)