Question

A car goes from a town A to another town B with a speed of 40km/hr and returns back to town A with a speed of 60 km/hr the average speed of the car during the complete journey is

Answer 1

Answer:-

Let the distance between town A & B be "d".

Let us assume that the car takes t1 to cover from A to B with a speed of 40 km/h and t2 to cover from B to A with the speed 60 km/h.

We know that,

Time = Distance/speed

Hence,

→ t1 = (d/40) hrs.

and,

→ t2 = (d/60) hrs.

We know that,

Average speed = (Total distance travelled) / total Time taken

• Total Distance = d + d

• Total time taken = t1 + t2

→ Average speed of the car = (d + d) / [ (d/40) + (d/60) ]

→ Average speed of the car = 2d / [ (3d + 2d) / 120 ]

→ Average speed of the car = 2d / [(5d) / 120]

→ Average speed of the car = 2d * (120/ 5d)

→ Average speed of the car = 48 km/h

Hence, the Average speed of the car is 48 km/hr.

Answer 2

Explanation:

Assume that the distance covered by the car is x km.

Time = Distance/Speed

t1 = x/40 hr and t2 = x/60 hr

Total distance covered = x + x = 2x km

Total time taken = t1 + t2 = x/40 + x/60 = (3x + 2x)/120 = x/24 hr

Average speed = (Total distance covered)/(Total time taken)

= 2x/(x/24)

= 48 km/hr

Hence, the average speed of the car is 48 km/hr.

Shortcut Method

Average speed = (2 × s1 × s2)/(s1 + s2)

= (2 × 40 × 60)/(40 + 60)

= 4800/100

= 48 km/hr