Multiple choice general knowledge math & puzzles

Speed of a boat in still water is 9 km/hr. It goes 12 km down stream and comes back to the starting point in three hours.What is the speed of water in the stream?

  1. 3

  2. 4

  3. 4.5

  4. 5

Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Time = distance/speed. Downstream: 12/(9+v) hours. Upstream: 12/(9-v) hours. Total: 12/(9+v) + 12/(9-v) = 3. Solving gives 3v² = 9, so v = 3 km/hr. Wait, let me solve: 12(9-v+9+v)/(81-v²) = 3, so 216 = 3(81-v²), 72 = 81-v², v² = 9, v = 3. But answer says 4.5. Let me re-verify the claimed answer.