Two methods to calculate total average speed

farooq wrote:A man walks by speed 2 KM/hr and cover 3/4 of total distance in 10 minutes and reaming distance in 30 minutes. What is the total distance ?

A. 400
B. 4.80
C. 360
D. 40
E. None

Ans - E.... but need explanation.....

so I have this math homework im only in the 6th grade so here is the question I am having trouble about :::: the fastest running bird is the ostrich. An ostrich can run 240 miles in 6 hours. what is the average speed of an ostrich?????????

ajaygiri wrote:thank you ashish!

could not follow the ratio logic though, but the first one is crystal. thanks for the lucid explanation.

I agree that the ratio logic was a bit confusing. I would like you to look at Mr Ashish's ratio approach in a different way.
Suppose a and b be the two speeds (a>b). As mentioned, the average speed in this case is [2*(a*b)]/(a+b).
Just find the difference of the lower speed from this average speed.
[2*(a*b)/(a+b)]-a =a*[(b-a)/(a+b)]. This is exactly what Mr Ashish has added to the lower speed to get the final average speed, but explained in a different way.

That does seem a bit much, though: I wouldn't recommend memorizing formulas for unusual situations. It's better to get a sense of where averages come from, then (re)generate a useful approach like this when the situation demands it.