A man travelled four equal distances of 7 km at the speeds of 8 km/hr, 16 km/hr, 24 km/hr and 32 km/hr respectively. What is his average speed?
Show answer & explanation
Correct answer: D
Explanation
The correct answer is Option 4 i.e. 384/25 km/hr.
We know that,
Time = Distance/Speed
Total distance covered by man = 4 × 7 = 28 km
Time taken by 1st 7 km at 8 km/hr
⇒ Time = 7/8
Time taken by 2nd 7 km at 16 km/hr
⇒ Time = 7/16
Time taken by 3rd 7 km at 24 km/hr
⇒ Time = 7/24
Time taken by 4th 7 km at 32 km/hr
⇒ Time = 7/32
Total time = 7/8 + 7/16 + 7/24 + 7/32
⇒ (84 + 42 + 28 + 21)/96 = 175/96
Now,
Average speed = Total distance/Total Time
⇒ Average speed = 28/175/96
⇒ 28 × 96/175 = 384/25 km/hr