P drives a car for 5 hours and after every hour he doubles the speed. In the fifth hour, his speed is 128 km/h. What is his average speed of the journey in km/h?

Let the speed in each hour be s
1
​
,s
2
​
,s
3
​
,s
4
​
,s
5
​
.
Since the speed doubles each hour:

s
5
​
=128 km/h

s
4
​
=
2
128
​
=64 km/h

s
3
​
=
2
64
​
=32 km/h

s
2
​
=
2
32
​
=16 km/h

s
1
​
=
2
16
​
=8 km/h

Since the duration of travel at each speed is exactly 1 hour, the distance covered in each hour is numerically equal to the speed:

Distance in 1st hour =8 km

Distance in 2nd hour =16 km

Distance in 3rd hour =32 km

Distance in 4th hour =64 km

Distance in 5th hour =128 km

Total distance covered:

Total Distance=8+16+32+64+128=248 km
Total time taken = 5 hours.

Average speed of the journey:

Average Speed=
Total Time
Total Distance
​
=
5
248
​
=49.6 km/h