A boat takes 3 hours more to travel a distance of 30 km upstream than it takes to travel the same distance downstream. If the speed of the boat in still water is three times the speed of the stream, then how much time will it take to travel a distance of 180 km in still water? 

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DSSSB PGT Chemistry (Female) Official Paper (Held On: 06 Jul, 2018 Shift 1)
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  1. 20 hours 
  2. 18 hours 
  3. 24 hours 
  4. 25 hours  

Answer (Detailed Solution Below)

Option 3 : 24 hours 
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Detailed Solution

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Given:

The boat takes 3 hours more to travel a distance of 30 km upstream than downstream.

Speed of the boat in still water = 3 × Speed of the stream.

Distance to be traveled in still water = 180 km.

Formula used:

Time = Distance ÷ Speed

Let the speed of the stream be x km/h.

Then, speed of the boat in still water = 3x km/h.

Upstream speed = (3x - x) = 2x km/h.

Downstream speed = (3x + x) = 4x km/h.

Calculations:

Time taken upstream - Time taken downstream = 3 hours

⇒ x = = 2.5 km/h

Speed of the boat in still water = 3x = 3 × 2.5 = 7.5 km/h

Time taken to travel 180 km in still water =

⇒ Time = 24 hours

∴ The correct answer is option (3).

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