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One-fifth of a journey is covered at a speed of 30 km/h, one-fourth of the journey at a speed of 25 km/h and the rest at 60 km/h. What is the average speed (in km/h, correct to one decimal place) for the whole journey?
1. 30.6
2. 40.5
3. 25.4
4. 38.7

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Best answer
Correct Answer - Option 4 : 38.7

Given:

1/5 of journey = 30kmph

1/4 of journey = 25kmph

(1 - 1/5 - 1/4 = 11/20) of journey = 60kmph

Formula:

Average speed = Total distance/Total time

Calculation:

Let total distance of journey be a km.

Time taken for 1/5 of journey = a/150 hours

Time taken for 1/4 of journey = a/100 hours

Time taken for 11/20 of journey = 11a/1200 hours

Total time taken for a km = a/150 + a/100 + 11a/1200 = 31a/1200 hours

∴ Average speed = a/(31a/1200) = 38.7kmph

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