Question

A stone is thrown straight upwards and it rises to a maximum height of 20m. With what speed was it thrown?

Answer 1

The maximum height

$$h_{\max}=\frac{v_0^2}{2g}$$

The initial speed

$$v_0=\sqrt{2gh_{\max}}=\sqrt{2*9.8*20}=19.8\:\rm m/s$$