Aptitude - Time and Distance - Discussion

Can we use the formula for avg speed, ie: 2xy/(x+y) ?This formula gives a different answer.

Can you explain how 9/2 hrs came ...
Can you explain the question ones again?

while using the formula for avg speed, as nisha said it gives a different answer..

Hi Nisha and Ranjith,



Average speed = 2xy/(x+y)



Where x = 64 km/hr and y = 80 km/hr.



Avg Speed = (2 x 64 x 80)/(64 + 80)



= 10240/144



= 71.111 km/hr



It is giving the correct answer only. Please make sure once again.





Hi Dinesh,



Your doubt is "How did 9/2 hrs come?".



1. The time taken for travelling first 160 km at 64 km/hr is = 160/64 = 2.5 hrs.



2. The time taken for travelling the next 160 km at 80 km/hr is = 160/80 = 2 hrs.



Therefore, total time taken (for 160 + 160 = 320 km) = 2.5 + 2 = 4.5 hrs.



We can write 4.5 hrs as 9/2 hrs. <-- I hope this is what you expect.





Now finding the avg speed.



1. Total distance = 320 km.



2. Total time taken = 4.5 hrs.



Avg Speed = 320/4.5 = 71.11111... km/hr



Note: Both methods are giving the same answer.

THANKS TO SUNDER WHO GIVES A GOOD SOLUTION

Hi Sundar, very nice explaination, thank you so much.

@sunder Thanks.

I have a doubt, can we do like this, the first 160 hrs is 64 km speed, and the second hr is 80, total 144. so 144/2 = to 72

Since distance is same,so
avg speed= 2xy/(x+y)
avg s= (2*64*80)/(64+80)
=71.11

Can you explain how 9/2 hours came ?