the current in a river is 3 mph . a boat can travel 21 mph in still water . how far up the river can the boat travel if the round trip is to take 10 hours ?
"upstream speed = 21 - 3 = 18 mph downstream speed = 21 + 3 = 24 mph d / 18 + d / 24 = 10 hours solving for d we get d = 102.8 answer : d"
divide(multiply(multiply(subtract(21, 3), add(21, 3)), 10), add(add(21, 3), subtract(21, 3)))