in a race u drove 1st lap with 40kmph and in the second lap at what speed u must drive so that ur average speed must be 80kmph.

Its impossible! if u drove the first lap in 40 kmph, its impossible that the 
average speed of both the laps is 80kmph.
for eg. consider one lap distance = 80km.
time req. to cover 1 lap = 80km/40kmph = 2 hrs.

 if the avg. speed is 80kmph, then the total time would have taken = 160kms/80kmph = 2 hrs.
same is the case with any other distance u consider. so the avg to be 80kmph is impossible