Numerical answer
A boat's speed in still water is 12 km/h. It travels 36 km downstream and then 24 km upstream, taking a total of 6 hours. Find the speed of the current in km/h.
Show answer and solution
Answer: 6
Let current speed = c km/h. Time = 36/(12+c) + 24/(12−c) = 6. Multiplying out and solving the resulting quadratic gives c = 6 or c = −4; only c = 6 is valid (speed must be positive and less than 12). Check: downstream speed = 18, time = 36/18 = 2h; upstream speed = 6, time = 24/6 = 4h; total = 6h. ✓
Saved on this device. Sign in or create a free account to keep it on every device.