Problem: a rower can row 5 km / h in still water . when the river is running at 2 km / h , it takes the rower 1 hour to row to big rock and back . how many kilometers is it to big rock ?
Rationale: "let x be the distance to big rock . time = x / 3 + x / 7 = 1 x = 21 / 10 = 2.1 km the answer is c ."
annotated_formula: multiply(divide(subtract(5, 2), add(add(5, 2), subtract(5, 2))), add(5, 2))