Problem: calculate the speed of a boat in still water ( in km / hr ) if in one hour , the boat goes 13 km / hr downstream and 10 km / hr upstream .
Rationale: "speed in still water = ( 13 + 5 ) 1 / 2 kmph = 9 kmph . answer : b"
annotated_formula: divide(add(13, 10), const_2)