$5\mathrm{.}2\mathrm{.}7$ Compute the critical flow depth in a trapezoidal channel

with a bottom width of $20$ ft a slope of $0\mathrm{.}0016,$ Manning's

$n$ of $0\mathrm{.}025,$ and side slopes of $2$:$1(h$:$v)$ for a discharge of $500$

cfs$.$