A boat can go 15 km upstream and 21 km downstream in 2 hours. If the speed of the stream is 3 km/h, then the time taken (in hours) by the boat to go 21 km upstream and 15 km downstream is :

Correct option is B

Given:

A boat can go 15 km upstream and 21 km downstream in 2 hours.

If the speed of the stream is 3 km/h, then the time taken (in hours)

by the boat to go 21 km upstream and 15 km downstream is

​Solution:

Let the speed of boat in still water = B

Let the speed of stream in still water = S

Speed of boat in downstream = B + S

Speed of boat in upstream = B - S

B + S $$= \frac{21}2 =$$​ 10.5km/h

B - S =$$\frac{ 15}2$$​= 7.5km/h

Solving after –

B= 9km/h and S = 1.5km/h

ATQ-

S =1.5 = 3km/h

B = 9 = 18 km/h

Now total taken time = $$\frac{21}{B-S}+\frac{15}{B+S}$$​​

=$$\frac{21}{15}+\frac{15}{21}$$​

=$$\frac{74}{35}$$​​

=2$$\frac{4}{35}$$​ hours