A tank has a constant cross-sectional area of $50 \mathrm{ft}^{2}$ and an orifice of constant cross-sectional area of $\frac{1}{2} \mathrm{ft}^{2}$ located at the bottom of the tank (see the accompanying figure).
If the tank is filled with water to a height of $h \mathrm{ft}$ and allowed to drain, then the height of the water decreases at a rate that is described by the equation
$$
\frac{d h}{d t}=-\frac{1}{25}\left(\sqrt{20}-\frac{t}{50}\right) \quad(0 \leq t \leq 50 \sqrt{20})
$$
Find an expression for the height of the water at any time $t$ if its height initially is $20 \mathrm{ft}$