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- Identify upstream or downstream.
- Apply the correct effective speed.
- Use distance/time relation.
Correct Answer: B
Correct Answer: B
Method / Topic: Boats and Streams
The answer uses the correct effective speed of the boat.
Short Explanation
Detailed Explanation
Correct Answer: B
Method / Topic: Boats and Streams
For downstream motion, add the speed of the stream to the speed of the boat in still water. Then use Time = Distance / Speed.
Speed of boat in still water = 12 km/h.
Speed of stream = 3 km/h.
Downstream speed = 12 + 3 = 15 km/h.
Distance = 45 km.
Time = 45 / 15 = 3 hours.
CHECK:
At 15 km/h, a boat covers 15 × 3 = 45 km in 3 hours. Hence the result is consistent.
EXAM TIP:
In Boats and Streams
Do not subtract the stream speed for downstream travel. The stream assists the boat, so the speeds must be added.
PRACTICAL EXAMPLE:
If a boat moves with a river current, the current increases its effective speed in the same way a moving walkway assists a person walking in its direction.
Therefore, the boat will take 3 hours to cover 45 km downstream.
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Learn + Solve: Shortcut, Trap & Fast Route Click to view shortcut, steps, and common mistake
Shortcut used, common trap, and the fastest solving route
- Identify upstream or downstream.
- Apply the correct effective speed.
- Use distance/time relation.
A boat moves at 12 km/h in still water and the stream flows at 3 km/h. How much time will the boat take to cover 45 km downstream?
Explanation
Correct Answer: B
Method / Topic: Boats and Streams
The answer uses the correct effective speed of the boat.
Short Explanation
Detailed Explanation
Correct Answer: B
Method / Topic: Boats and Streams
For downstream motion, add the speed of the stream to the speed of the boat in still water. Then use Time = Distance / Speed.
Speed of boat in still water = 12 km/h.
Speed of stream = 3 km/h.
Downstream speed = 12 + 3 = 15 km/h.
Distance = 45 km.
Time = 45 / 15 = 3 hours.
CHECK:
At 15 km/h, a boat covers 15 × 3 = 45 km in 3 hours. Hence the result is consistent.
EXAM TIP:
In Boats and Streams
Do not subtract the stream speed for downstream travel. The stream assists the boat, so the speeds must be added.
PRACTICAL EXAMPLE:
If a boat moves with a river current, the current increases its effective speed in the same way a moving walkway assists a person walking in its direction.
Therefore, the boat will take 3 hours to cover 45 km downstream.
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