A man’s average driving speed for 9 hours is 72 km/h. During the first 3 hours of his journey, he drove at 54 km/h, and for the next 2 hours and 15 minutes, he drove at a speed of 68 km/h. He maintained a speed of 90 km/h for another 3 hours. What was his speed (in km/h) for the last 45 minutes?
Show answer & explanation
Correct answer: D
Explanation
The correct answer is option 4 i.e. 84.
Total time = 9 hours
Average speed = 72 km/h
Total distance = 72 × 9 = 648 km
First 3 hours at 54 kmph, Distance = 3 × 54 = 162 km
Next 2 hrs 15 mins at 68 kmph = 2.25 hrs
Distance = 2.25 × 68 = 153 km
Next 3 hours at 90 kmph , Distance = 3 × 90 = 270 km
Total distance so far : 162 + 153 + 270 = 585 km
Time left : 9 − (3 + 2.25 + 3) = 0.75 hours = 45 minutes
Distance remaining = 648 − 585 = 63 km
Speed = 63/0.75 = 84 km/h