Directions: A boatman takes his boat upstream from place A to place B, where AB = 21 km. Then, he returns to place A. He takes 10 hours in all. The time taken by him to row 7 km downstream is equal to the time taken by him to row 3 km upstream. (Assume he rows with the same speed upstream as downstream.) Find the velocity of the stream.

Because the time to row 7 kilometers downstream equals the time to row 3 kilometers upstream, the ratio of downstream speed to upstream speed is 7 to 3. Let the downstream and upstream speeds be 7x and 3x, so the total time for the 21-kilometer round trip is 21 divided by 7x plus 21 divided by 3x, which equals 10 hours. Solving this gives x as 1 kilometer per hour, making the downstream speed 7 kilometers per hour and the upstream speed 3 kilometers per hour. The velocity of the stream is half the difference of these two speeds, resulting in 2 kilometers per hour.

• Thanks
• Wrong explanation