A bullet fired horizontally over level ground hits the ground in 0.5 second. If it had been fired with twice the speed in the same direction, it would have hit the ground in

In 0.5 seconds.

Explanation:

The time would be the same because it only depends on the height and the vertical component of the initial velocity. This is of course because each direction must be treated independently. Since between both cases only the horizontal speed changes, the height is the same and the vertical component of the initial velocity is null for both, the time to fall is the same.