relative motion Questions

Two bullets are fired horizontally and simultaneously towards each other from the rooftops of two buildings (building distance being $200\text{m}$ apart and being of the same height of $200\text{m}$) with the same velocity of $25\text{m/s}$. When and where will the two bullets collide? ($g=10{\text{m/s}}^2$)

A person standing on the floor of an elevator drops a coin. The coin reaches the floor in time $t_1$ if the elevator is moving uniformly and time $t_2$ if the elevator is stationary. Then: