Rajinder drives his car at an average speed of 40 km per hour to cover half of his total distance and thereafter he increases his speed to 60 km per hour. He covers his journey in 7 hours. What is the average speed of Rajinder in this journey?
Show answer & explanation
Correct answer: B
Explanation
The correct answer is option 2 i.e. 48 km per hour.
Short method:
Average speed = 2S1S2/(S1 + S2)
= 2 × 40 × 60/(40 + 60) = 4800/100 = 48 km/h
Alternate method:
Let the distance travels by Rajinder be 2D
Total time taken = 7 hours
Initial speed of car = 40 km/h
Increased speed of car = 60 km/h
Distance = Speed × Time
According to the question
⇒ D/40 + D/60 = 7
⇒ 5D = 7 × 120
⇒ D = 168 km
⇒ Total distane = 2D = 2 × 168 = 336 km
Hence, the average speed of Rajinder in this journey = 336/7 = 48 km/h