A bus travels 90 km at 45 km/h, 120 km at 60 km/h, and 150 km at 75 km/h. What is the average speed of the bus for the entire journey?
- AOption (a)
66 km/h
- BOption (b)
55 km/h
- COption (c)
60 km/h
- DOption (d)
70 km/h
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Correct answer
60 km/h
Why this answer is correct
Shortcut Trick
Identify Total Distance = 90 + 120 + 150 = 360 km.
Calculate Total Time = (90 ÷ 45) + (120 ÷ 60) + (150 ÷ 75) = 2 + 2 + 2 = 6 hours.
Average Speed = Total Distance ÷ Total Time = 360 ÷ 6 = 60 km/h.
∴ The correct answer is 60 km/h.
Alternate Method
Given:
Distance1 (d1) = 90 km, Speed1 (s1) = 45 km/h
Distance2 (d2) = 120 km, Speed2 (s2) = 60 km/h
Distance3 (d3) = 150 km, Speed3 (s3) = 75 km/h
Formula Used:
Average Speed = Total Distance ÷ Total Time
Calculations:
⇒ Time taken for first part (t1) = 90 ÷ 45 = 2 hours
⇒ Time taken for second part (t2) = 120 ÷ 60 = 2 hours
⇒ Time taken for third part (t3) = 150 ÷ 75 = 2 hours
⇒ Total Distance = 90 + 120 + 150 = 360 km
⇒ Total Time = 2 + 2 + 2 = 6 hours
⇒ Average Speed = 360 ÷ 6 = 60 km/h
∴ The correct answer is 60 km/h.
Additional Information
Average Speed for Equal Distances
If a journey is divided into two equal distances with speeds x and y, the Average Speed = 2xy ÷ (x + y).
Unit Conversion
To convert speed from km/h to m/s, multiply by 5/18. To convert from m/s to km/h, multiply by 18/5.
Relative Speed
When two objects move in the same direction, relative speed is s1 − s2. In opposite directions, it is s1 + s2.