Single correct option
A boat covers 36 km downstream and 24 km upstream in 6 hours. It also covers 48 km downstream and 16 km upstream in 6 hours. What is the speed of the boat in still water (in km/h)?
- A. 8
- B. 12
- C. 10
- D. 9
Show answer and solution
Answer: (C) 10
Let downstream speed = d and upstream speed = u.
36/d + 24/u = 6 and 48/d + 16/u = 6.
Subtracting: 12/d = 8/u, so u = 2d/3 and 24/u = 36/d. The first equation becomes 72/d = 6, so d = 12 and u = 8.
Speed in still water = (12 + 8)/2 = 10 km/h (stream = 2 km/h).
Trap: 12 and 8 are the downstream and upstream speeds, not the boat's speed.
36/d + 24/u = 6 and 48/d + 16/u = 6.
Subtracting: 12/d = 8/u, so u = 2d/3 and 24/u = 36/d. The first equation becomes 72/d = 6, so d = 12 and u = 8.
Speed in still water = (12 + 8)/2 = 10 km/h (stream = 2 km/h).
Trap: 12 and 8 are the downstream and upstream speeds, not the boat's speed.
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