A car travels first 30 km with a uniform speed of 60 km $h^{- 1}$ and then next 30 km with a uniform speed of 40 km $h^{- 1}$ . Calculate:

(i) the total time of journey,
(ii) the average speed of the car.

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Solution

(i) Time taken for first part of the journey, $t_{1} = \frac{d_{1}}{v_{1}} = \frac{30}{60} h = 0.5 h$

Time taken for second part of the journey, $t_{2} = \frac{d_{2}}{v_{2}} = \frac{30}{40} h = 0.75 h$

Total time, $t = t_{1} + t_{2} = 0.5 + 0.75 = 1.25 h$

(ii) Average speed $= \frac{Total distance}{Total time} = \frac{30 + 30}{1.25} = 48 k m p h$

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