SSC CGL 2021 PYQ — Question 61

 The correct answer is Option 2 i.e 4.

Let the speed of the boat and the stream be 'a' and 'b' km/hr.

Speed in case of downstream = (a + b)km/hr

Speed in case of downstream = (a - b) km/hr

Suppose it takes 't' time to row a distance of 8 km downstream and can row 6 km upstream.

According to the question,

⇒ 8/(a + b) = 6/(a - b)

⇒ 8a - 8b = 6a + 6b

⇒ 2a = 14b

⇒ a = 7b

Let a = 7x and b = x

He rows 24 km upstream and the same distance downstream in $1\frac34$ hours,

 $\Rightarrow \frac{24}{a+b} + \frac{24}{a-b}=\frac{7}{4} \\ \Rightarrow \frac{24}{8x} + \frac{24}{6x}=\frac{7}{4} \\ \Rightarrow \frac{3}{x} + \frac{4}{x} = \frac74$

⇒ x = 4