If one bullet is dropped from a certain height and another is fired from a gun horizontally from the same height which one will hit the ground first or will they hit at the same time?

If the ground is flat, and both bullets are released at the same time from the same height, then they both hit the ground at the same time.

The horizontal motion of the one from the gun has no effect on its vertical motion.

multiply 2kg by latent heat of vap

d. air pressure decreases as altitude increases.