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A speed of the boat in still water is 15km/hr and speed of the stream is 5km/hr, the boat travels equal distance for upstream and downstream in 9 hours.  find the total distance travelled by boat in both upstream and downstream.


1. 150 Km
2. 160 Km
3. 200 Km
4. 120 Km
5. 240 Km

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Correct Answer - Option 4 : 120 Km

Given:

The speed of the boat in the still water = 15 km/hr

The speed of the stream = 5 km/hr

Let the Distance travelled by boat in upstream and downstream be d

Total time is taken by the boat to cover both upstream and downstream = 9 hrs

Formula Used:

Distance = Speed × Time

Upstream boat speed (x-y) = speed of the boat in still water – the speed of the stream

Downstream boat speed (x+y) = speed of the boat in still water + speed of the stream

The formula for this method:   

t = \([\frac{d}{{(x + y)}} + \frac{d}{{(x - y)}}]\)

d is the distance

x is the speed of the boat in still water.

Y is the speed of the stream

t is the time taken

Calculation:

Substituting the values in the formula

Speed of the boat in upstream (x - y) = 15 – 5 ⇒10 Km/hr

Speed of the boat in downstream (x + y) = 15 + 5 ⇒ 20 km/hr

Distance travelled by upstream and downstream 

\([\frac{d}{{10}} + \frac{d}{{20}}]\) ⇒ 9 

3D = 9 × 20

D ⇒\([\frac{{180}}{3}]\) = 60 Km

∴ Total distance both upstream and downstream = 60 + 60 ⇒ 120 km

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