$5.(9\mathrm{.}10)$ An electric fan is turned off, and its angular velocity de$-$creases uniformly from $510$ rev$/$min to $200$ rev$/$min in $4\mathrm{.}50$ s$.$ a$.$ Find the angular acceleration in rev$/$s$2$ and the number of revolutions made by the motor in the $4\mathrm{.}50$ s interval. rev b$.$ How many more seconds are required for the fan to come to rest if the angular acceleration remains constant at the value calculated in part $($a$)?$