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