Question

A body covers half of its journey with a speed of 'a' m/s and the other half with a speed of 'b' m/s. Calculate the average speed of the body during the whole journey.

Answer 1

answer - 2*a*b/(a+b)

Answer 2

Let the distance be d,
For distance (1/2)d;
Speed s1=a m/s and 
The time taken to cover this distance will be t1=(1/2) d÷s1=(1/2)d÷a seconds.
For the another half  distance (1/2)d;
Speed s2= b m/s and
The time taken to cover this ditance will be t2=(1/2)d÷s2=(1/2)d÷b seconds
So the total distance = (1/2)d+ (1/2)d=d
The average Speed to travel the distance d= d÷[ { (1/2) d ÷ a} + {(1/2)d}÷b]    
On solving the above we get :-

Speed S=ab/(a+b) metre per second