On a journey across Delhi, a taxi average 30 kmph for 60% of the distance, 20 kmph for 20% of it and 10 kmph for the remainder, the average speed for the whole journey is?

Let total journey = y km
Total time taken = time taken during 60% of distance + time taken during 20% of distance + time taken in remaining distance
= [( 60y / 100 ) / 30] + [( 20y / 100 ) / 20] + [( 20y / 100 ) / 10]
= ( y / 50 + y / 100 + y / 50 )hrs
= y/20 hours.
Average speed = total distance traveled / total time taken.
= [y / ( y/20 )] km/hr
=20 km/hr.