Question

A man rows a boat 18km in 4 hours downstream and returns upstream in 12 hours. the speed of the stream (in km per hour) is:
a. 1 b. 1.5
c. 2 d. 1.75​

Answer 1

Answer:

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hence speed of stream is 1.5 km/hr

Answer 2

The speed of the stream is 1.5 Km/hour. Thus Option B is correct

Solution:

Given distance = 18 km

Time taken for downstream = 4 hours

Time taken for upstream = 12 hours

Speed of stream = ?

Let the speed of the boat be "x" and that of stream be "y"

When moving downstream:

$\text { Speed of boat }+\text { speed of stream }=\frac{\text { Distance }}{\text { Time }}$

x + y = 18 ÷ 4 = 4.5 km/hour    …(1)

When moving Upstream:

$\text { Speed of boat-speed of stream }=\frac{\text { Distance }}{\text { Time }}$

x – y = 18 ÷ 12 = 1.5 Km/hour    …(2)

Adding equation (1) and (2) we get,

2x = 6  

x = 6÷2 = 3

And put value of x in (1) we get  

3 + y = 4.5

y = 1.5 km/hour

The speed of the stream is 1.5 Km/hour  and Option B is correct

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