$16\mathrm{.}20$ Select simply supported timber beams $($S$4$S$)$ for the following uniformly distributed line loads and spans. Deflection is not to exceed span$/240.$

$($a$)$ Douglas fir, $400$ lb$/$ft$,22-$ft span

$($b$)$ hem$-$fir, $400$ lb$/$ft$,18-$ft span