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Question
A car travels the first 30 km with a uniform speed of 60 km h-1 and the next 30 km with a uniform speed of 40 km h-1. Calculate :
The total time of journey,
The average speed of the car.
Solution
For the first 30 km travelled, speed = 60 km/h.
Thus time taken (t1) = Distance / speed
= (30/60) h-1
= 0.5 h-1 or 30 min.
For the next 30 km travelled, speed = 40 km/h
Thus time taken (t2) = Distance/speed
= (30/40) h-1
= 0.75 h-1 or 45 min.
(i) Total time = (30 + 45) min
= 75 min or 1.25 h.
(ii) Average speed of the car = Total distance travelled/total time taken = `(60 "km")/(1.25 "hr")` = 48 km h-1
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