A man throws balls with the same speed vertically upwards one after the other at an interval of 2 seconds. What should be the speed of the throw so that more than two balls are in the sky at any time? (Given g=9.8m/s2 )

Interval of ball throw=2sec If we want that minimum three (more than two) balls remain in air then time of flight of first ball must be greater than 4 sec , i.e.,T>4 sec Or 2ug>4 sec  or u>19.6m/s Foru=19.6 . First ball will just strike the ground (in sky), second ball will be highest point (in sky). Third ball will be at the point of projection or at ground (not in sky).