Question

A vehicle moving a speed of first half distance take 80kilometer per hour and last half distance take 120km/hr. find the average speed?

Answer 1

100 km/h
I think it is correct....

Answer 2

For same distance covered,
Avg speed = 2*a*b/a+b... where a & b are respective speeds for distances. 
Avg. speed = 2*120*80/120+80 = 2*120*80/200 = 96 km/hr.

OR
Let tot dist = D
Avg. speed = Total dist/Total time
Time for 1st half distance = (D/2) / 80 = D/160
Time for next half distance = (D/2) /120 = D/240
Avg speed= D/(D/160 + D/240) = D/D(1/160 + 1/240) = 1/(6+4)/960 = 960/10 = 96 km/hr
Hence, avg speed= 96 km/hr
Hope it helps.