A car drifting at the speed of 15 Kmph, decides to boost its speed by 100% in exactly the middle of the journey. Calculate the average speed of the car after completion of the journey.

Option 1 : 20 km/hr

**Calculation **

Let the total distance traveled throughout the journey be D.

Distance traveled in the first half of the journey be x km and another half be y km.

x km distance was covered by the speed of 15 km/h

y km distance was covered by the speed of 100% more than the speed of x i.e. 30 km/h.

Total distance travelled = x + y km

⇒ 2x km

As x = y as it is mentioned in the middle of the journey.

Total time = t_{1} + t_{2}

Where, t_{1} = x/15, t_{2} = y/30 (using time = distance/ speed)

Average speed = total distance/ total time

⇒ 2x/(x/15 + x/30)

= 20 km/h

∴ The average speed at the completion of the journey is 20 km/h.

Average speed = 2ab/(a+b)

Where a and b are the speed traveled in the two equal halves

Average speed = (2 × 15 × 30)/ (15 + 30)

⇒ 20 Kmph

**∴ The average speed at the completion of the journey is 20 km/h.**