A car travels first $30 k m$ with a uniform speed of $60 k m p h$ and then next $30 k m$ with a uniform speed of $40 k m p h$ . Find the average speed of the car.

24 k m p h

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48 k m p h

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72 k m p h

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96 k m p h

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Solution

The correct option is B $48 k m p h$
Time for first part $= t_{1} = \frac{30}{60} h r = 0.5 h r = 30 m i n$

Time for second part $= t_{2} = \frac{30}{40} h r = 0.75 h r = 45 m i n$

$∴$ total time $= t_{1} + t_{2} = (30 + 45) m i n = 75 m i n = \frac{75}{60} h r = 1.25 h r$

And total distance $= s_{1} + s_{2} = (30 + 30) k m = 60 k m$

So, average speed $= \frac{s_{1} + s_{2}}{t_{1} + t_{2}} = \frac{60}{1.25} k m p h = 48 k m p h$

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