An automobile went up a hill at a speed of $10 k m$ an hour and down the same distance at a speed of $20 k m$ an hour. The average speed for the round trip was

12 \frac{1}{2} k m / h r

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13 \frac{1}{3} k m / h r

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14 \frac{1}{2} k m / h r

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15 k m / h r

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None of these

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Solution

The correct option is E $13 \frac{1}{3} k m / h r$
$t_{1} = \frac{d}{10}, t_{2} = \frac{d}{20}, t_{1} + t_{2} = (\frac{1}{10} + \frac{1}{20})$ ;
$∴ A = \frac{2 d}{d [(1 / 10) + (1 / 20)]} = \frac{40}{3} = 13 \frac{1}{3} k m / h r$ .

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An automobile went up a hill at a speed of 10 km an hour and down the same distance at a speed of 20 km an hour. The average speed for the round trip was

The correct option is E 1313km/hrt1=d10,t2=d20,t1+t2=(110+120);∴A=2dd[(1/10)+(1/20)]=403=1313 km/hr.

An automobile went up a hill at a speed of $10 k m$ an hour and down the same distance at a speed of $20 k m$ an hour. The average speed for the round trip was

The correct option is E $13 \frac{1}{3} k m / h r$
$t_{1} = \frac{d}{10}, t_{2} = \frac{d}{20}, t_{1} + t_{2} = (\frac{1}{10} + \frac{1}{20})$ ;
$∴ A = \frac{2 d}{d [(1 / 10) + (1 / 20)]} = \frac{40}{3} = 13 \frac{1}{3} k m / h r$ .