Question

A and B participated in 150 m race. A gives B a start of 15 m and still beats him by 9 seconds. If A runs at a speed of 12 km/h, find the speed of B in km/h?
1. 11 km/h
2. 7 km/h
3. 8 km/h
4. 9 km/h

Answer 1

Correct Answer - Option 4 : 9 km/h

Given:

Distance of race = 150 m

Speed of A = 12 km/h

B ahead of A = 15 m

A beats B by = 9 sec

Concept:

Since, B gets a start of 15 m and A beats him by 9 seconds, it means that the time in which A finish the race, B taken extra 9 seconds to reach the finish line.

Formula used:

km/h to m/s conversion = a km/h × 5/18

m/s to km/h conversion = a m/s × 18/5

Calculation:

Speed of A in m/s = 12 × (5/18) m/s

= 10/3 m/s

Time taken by A to finish the race = (150 × 3)/10 sec

= 45 sec

Time taken by B to finish the race after A started running = 45 + 9 sec

= 54 sec

∵ B covered (150 - 15) m in 54 seconds

∴ Speed of B = 135/54 m/s

= 5/2 m/s

∴ Speed of B in km/h = (5/2) × (18/5) km/h

= 9 km/h