A man drops a ball downside from the roof of a tower of height $400$ $\mathrm{m}$. At the same time, another ball is thrown upside with a velocity $50\mathrm{ }\mathrm{m} {\mathrm{s}}^{-1}$ from the surface of the tower, then they will meet at what height from the surface of the tower?

A man throws balls with the same speed vertically upwards, one after the other, at an interval of $2 \mathrm{s}$. What should be the speed of the throw, so that more than two balls are in the sky at any time? (Given, $g=10 \mathrm{m} {\mathrm{s}}^{-2}$)

A person standing on the roof of a $40\mathrm{ }\mathrm{m}$ high tower, throws a ball vertically upwards with speed $10\mathrm{ }\mathrm{m} {\mathrm{s}}^{-1}$. Two seconds later, he throws another ball again in vertical direction (use $g=10\mathrm{ }\mathrm{m} {\mathrm{s}}^{-2}$). Both the balls hit the ground simultaneously.