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P and Q are two cities. A boy travels on cycle from P to Q at a speed of 20 km per hr. and returns at the rate of 10 km per hr. Find his average speed for the whole journey.
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13 2 kmph 3 -
12 1 kmph 3 -
13 1 kmph 3 -
12 2 kmph 3
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Correct Option: C
Let the speed between P and Q be x km.
| Then time taken to cover x km. P to Q = | hrs. | |
| 20 |
Time taken to cover x km from Q to P at 10 km per hr. P to Q
| = | hrs. | |
| 10 |
∴ Total distance covered
= x + x = 2x km.
Time taken to cover 2x km.
| = | + | = | = | hrs. | ||||
| 20 | 10 | 20 | 20 |
| ∴ Average Speed = | = | ||
| 3x/20 | 3x |
| = | km per hr. | |
| 3 |
| = 13 | km per hr. | |
| 3 |
Second Method : Here, x = 20 km per hr.
y = 10 km per hr
∴ Average Speed [∵ Distance is same]
| = | = | ||
| x + y | 20 + 10 |
| = | = | = 13 | km per hr. | |||
| 30 | 3 | 3 |
