SSC CHSL 2022 PYQ — Question 51

The correct answer is Option 2 i.e. 40 km/h.

Given:

Speed in journey from A to B = 60 km/h.

Total average speed = 48 km/hr

Formula Used:

Distance = Speed × Time

$Average \ speed = {Total\ Distance \over Total\ Time}$

Explanation:

Let the distance between A and B be D km and the speed of Ruksana while travelling from B to A be 'x' km/hr.

Now Total distance = 2D

Time taken while travelling from A to B = ${D \over 60}$ hr

Similarly, Time taken while travelling from B to A = ${D \over x}$ hr.

Now, Total Time = $({D \over 60} + {D \over x})$ hr

So, Average speed, $48 = {2D \over {D \over 60} + {D \over x}}$

⇒ $48({D \over 60} + {D \over x}) = 2D$

⇒ $24({1 \over 60} + {1 \over x}) = 1$

⇒ $({1 \over 60} + {1 \over x}) = {1 \over 24}$

⇒ ${1 \over x} = {1 \over 24} - {1 \over 60}$

⇒ ${1 \over x} = {3 \over 120}$

⇒ ${1 \over x} = {1 \over 40}$

⇒ x = 40

Hence the speed of Ruksana while travelling from B to A is 40 km/hr.