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- Identify upstream or downstream.
- Apply the correct effective speed.
- Use distance/time relation.
Correct Answer: C
Correct Answer: C
Method / Topic: Boats and Streams
The answer uses the correct effective speed of the boat.
Short Explanation
Detailed Explanation
Correct Answer: C
Method / Topic: Boats and Streams
The speed of the boat in still water is the average of its downstream and upstream speeds.
Downstream speed = 18 km/h.
Upstream speed = 10 km/h.
Speed in still water = (18 + 10) / 2.
= 28 / 2.
= 14 km/h.
CHECK:
If the boat speed is 14 km/h, then the stream speed is (18 - 10)/2 = 4 km/h.
Downstream speed = 14 + 4 = 18 km/h.
Upstream speed = 14 - 4 = 10 km/h.
Both match the given values.
EXAM TIP:
For a direct downstream-upstream pair:
Boat speed = (Downstream + Upstream)/2.
Stream speed = (Downstream - Upstream)/2.
Do not take the difference of downstream and upstream speeds as the boat's speed. Half of the difference gives the stream speed.
PRACTICAL EXAMPLE:
The same averaging idea works whenever one observed speed is increased by a fixed effect and the other is reduced by the same effect.
Therefore, the speed of the boat in still water is 14 km/h.
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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.
The downstream speed of a boat is 18 km/h and its upstream speed is 10 km/h. What is the speed of the boat in still water?
Explanation
Correct Answer: C
Method / Topic: Boats and Streams
The answer uses the correct effective speed of the boat.
Short Explanation
Detailed Explanation
Correct Answer: C
Method / Topic: Boats and Streams
The speed of the boat in still water is the average of its downstream and upstream speeds.
Downstream speed = 18 km/h.
Upstream speed = 10 km/h.
Speed in still water = (18 + 10) / 2.
= 28 / 2.
= 14 km/h.
CHECK:
If the boat speed is 14 km/h, then the stream speed is (18 - 10)/2 = 4 km/h.
Downstream speed = 14 + 4 = 18 km/h.
Upstream speed = 14 - 4 = 10 km/h.
Both match the given values.
EXAM TIP:
For a direct downstream-upstream pair:
Boat speed = (Downstream + Upstream)/2.
Stream speed = (Downstream - Upstream)/2.
Do not take the difference of downstream and upstream speeds as the boat's speed. Half of the difference gives the stream speed.
PRACTICAL EXAMPLE:
The same averaging idea works whenever one observed speed is increased by a fixed effect and the other is reduced by the same effect.
Therefore, the speed of the boat in still water is 14 km/h.
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