Quantitative Aptitude – Practice – More on average speed!

PSA_LEVEL-UP_QNR31. Monto travels his whole journey at four different speeds. Quarter of his journey he travels at lowest speed of 20 km/h. 1/3rd of the rest he speeds up and travels at 25 km/h. Half of the rest he increases his speed again by 5 km/hr. After looking at his watch and deciding to reach on time, he finally increases his speed by one-third. What is his average speed?

a) 13 78/89 km/h
b) 26 86/89 km/h
c) 12 km/h
d) 28 km/h


Consider the total distance to be travelled is 24D.
Time taken to cover first leg of journey or one fourth of 24D = 6D/20
Distance remaining= 18D

Time taken to cover second leg of journey or one third of remaining journey or 6D = 6D/25
Now, distance remaining= 18D – 6D = 12D
New speed = 25km/h + 5 km/hr = 30km/hr

Time taken to cover third leg of journey or half of remaining journey or 6D = 6D/30
Now, distance remaining= 12D – 6D = 6D
New speed = 30km/hr + 1/3rd of 30km/hr = 40km/hr

Time taken to cover last leg of journey = 6D/40

Average speed = Total distance covered/Total time taken
= 24D/[6D/20+6D/25+6D/30+6D/40]
=2400/89. Option b.

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