A man’s average driving speed for 9 hours is 72km/h. During the first 3 hours of his journey, he drove at 54km/h, and for the next 2 hours and 15 minutes, he drove at a speed of 68km/h. He maintained a speed of 90km/h for another 3 hours. What was his speed (in km/h) for the last 45 minutes?

Total distance covered in 9 hours:
Total Distance=9×72=648km
Let's find the distance covered in each part of the journey:

First part (3 hours at 54km/h):

D
1
​
=3×54=162km
Second part (2 hours 15 minutes = 2.25 hours at 68km/h):

D
2
​
=2.25×68=153km
Third part (3 hours at 90km/h):

D
3
​
=3×90=270km
Sum of distances for the first 3 parts:

Sum=162+153+270=585km
Remaining distance to be covered in the last 45 minutes (
60
45
​
=0.75 hours):

Remaining Distance=648−585=63km
Speed required for the last 45 minutes:

Speed=
Time
Distance
​
=
0.75
63
​
=84km/h