Why would you throw like that?

Classical Mechanics Level 3

Standing on a building $80\text{m}$ high, I threw a stone vertically upwards at a velocity of $20\text{m/s}$ . Then find the time(in seconds) it takes to reach the ground.

Details and assumption:-

Take $\text{g}$ (acceleration due to gravity) as $10{\text{m/s}}^2$ .
Give your answer to two decimal places.