DST

Ok i think you've posted in the wrong section. This is the DS section

Average Speed = Total Distance/Total Time.
Total time = Total distance/average speed.

To find the average speed of the last 20 miles, we need the total time it took to cover the last 20 miles.

Time taken to cover last 20 miles = time taken to cover 40 miles - time taken to cover first 20 miles.

= 40/60-20/50
=4/15 hours.

Hence average speed of the last 20 miles = 20/(4/15) = 75mph.

Yes there is a formula but what Mals did works just fine also.


If 2 equal distances are covered at different speeds then

average speed = 2 * speed1 * speed2 /(speed1 + speed2)

Given three equal distances are covered by different speeds lets say x,y,z

then average speed = 3xyz / xy + yz + zx


Using formula 1 above

2 * 50 * speed2 / (50+ speed2) = 60

100speed2 = 3000 + 60speed2

speed2 = 75


Regards,
CR

cramya wrote: average speed = 2 * speed1 * speed2 /(speed1 + speed2)

Given three equal distances are covered by different speeds lets say x,y,z

then average speed = 3xyz / xy + yz + zx

Hey cramya thanks for sharing the formula Im looking at it for the first time.

No probs Mals. Hope u r doing well...

Regards,
CR

Here comes the logic behind the formula

So using the formula above:

2/Vav = 1/v1 + 1/v2

2/60 = 1/50 + 1/X

1/x = 1/30-1/50

x=75