Practice Questions
Concept Explanation
A boat's speed in still water combines with the stream. Downstream motion adds the stream; upstream subtracts it.
Key Formulas
- Downstream speed = Boat speed + Stream speed
- Upstream speed = Boat speed − Stream speed
- Boat speed in still water = (Downstream + Upstream) / 2
- Stream speed = (Downstream − Upstream) / 2
Example
Downstream 20 km/h, upstream 12 km/h. Boat = (20 + 12)/2 = 16 km/h; stream = 4 km/h.
Common Trap
Add stream speed only when going downstream. Subtract when going upstream.
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Q1 Easy
The speed of a boat in still water is 15 km/h and the speed of the stream is 3 km/h. What is the speed of the boat while going downstream?
ExplanationDownstream speed = 15 + 3 = 18 km/h. -
Q2 Easy
The speed of a boat in still water is 15 km/h and the speed of the stream is 3 km/h. What is the speed of the boat while going upstream?
ExplanationUpstream speed = 15 − 3 = 12 km/h. -
Q3 Easy
A boat travels downstream at 24 km/h and upstream at 16 km/h. What is the speed of the boat in still water?
ExplanationBoat speed in still water = (24 + 16) ÷ 2 = 20 km/h. -
Q4 Easy
A boat travels downstream at 24 km/h and upstream at 16 km/h. What is the speed of the stream?
ExplanationStream speed = (24 − 16) ÷ 2 = 4 km/h. -
Q5 Medium
A boat can move at 10 km/h in still water. If the stream flows at 2 km/h, how long will the boat take to cover 36 km downstream?
ExplanationDownstream speed = 12 km/h. Time = 36 ÷ 12 = 3 hours. -
Q6 Medium
A boat can move at 12 km/h in still water. If the stream flows at 4 km/h, how long will the boat take to cover 32 km upstream?
ExplanationUpstream speed = 8 km/h. Time = 32 ÷ 8 = 4 hours. -
Q7 Easy
A boat travels downstream at 18 km/h and upstream at 10 km/h. What is the speed of the stream?
ExplanationStream speed = (18 − 10) ÷ 2 = 4 km/h.
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Q8 Hard
A boat moves at 8 km/h in still water and the stream flows at 2 km/h. The boat makes a round trip of 40 km, travelling 20 km downstream and 20 km upstream. How long does the entire journey take?
ExplanationDownstream: 20 ÷ 10 = 2 h. Upstream: 20 ÷ 6 ≈ 3.33 h. Total ≈ 5.33 h ≈ 5.5 h. -
Q9 Easy
A man can row at 6 km/h in still water and the stream flows at 2 km/h. What is his speed while rowing downstream?
ExplanationDownstream speed = 6 + 2 = 8 km/h. -
Q10 Easy
The speed of a boat in still water is 14 km/h and the speed of the stream is 3 km/h. What is the speed of the boat while going upstream?
ExplanationUpstream speed = 14 − 3 = 11 km/h. -
Q11 Medium
A boat travels downstream at 30 km/h and upstream at 18 km/h. What is the speed of the boat in still water?
ExplanationBoat speed in still water = (30 + 18) ÷ 2 = 24 km/h. -
Q12 Hard
The speed of a boat in still water is 16 km/h and the stream flows at 4 km/h. The boat travels 30 km downstream and then returns 30 km upstream. What is the total time for the round trip?
ExplanationDownstream: 30 ÷ 20 = 1.5 h. Upstream: 30 ÷ 12 = 2.5 h. Total = 4 h. -
Q13 Medium
The speed of the stream is 5 km/h. If the downstream speed of a boat is 25 km/h, what is its upstream speed?
ExplanationStream = (25 − u) ÷ 2 = 5, so upstream speed u = 15 km/h. -
Q14 Medium
A boat can move at 9 km/h in still water. If the stream flows at 1 km/h, how long will the boat take to cover 40 km upstream?
ExplanationUpstream speed = 8 km/h. Time = 40 ÷ 8 = 5 hours.
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DS1 Easy
What is the downstream speed of the boat?
Statement IThe speed of the boat in still water is 15 km/h.
Statement IIThe speed of the stream is 3 km/h.
ExplanationBoth statements are needed: downstream speed = 15 + 3 = 18 km/h. -
DS2 Easy
What is the speed of the boat in still water?
Statement IThe downstream speed of the boat is 24 km/h.
Statement IIThe upstream speed of the boat is 16 km/h.
ExplanationBoth downstream and upstream speeds are needed: boat speed = (24 + 16) ÷ 2 = 20 km/h.
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DS3 Easy
What is the speed of the stream?
Statement IThe downstream speed of the boat is 20 km/h.
Statement IIThe upstream speed of the boat is 12 km/h.
ExplanationBoth statements are needed: stream speed = (20 − 12) ÷ 2 = 4 km/h. -
DS4 Medium
How long will the boat take to cover 36 km downstream?
Statement IThe speed of the boat in still water is 10 km/h.
Statement IIThe speed of the stream is 2 km/h.
ExplanationBoth statements are needed: downstream speed = 12 km/h. Time = 36 ÷ 12 = 3 hours.