$($b$)$ An analogue cellular system has a total of $40$ MHz of bandwidth and uses two $15-kHz$ simplex $($one$-$way$)$ channels to provide full duplex voice and control channels. What is the number of channels available per cell for a frequency reuse factor of:

$($i$)4$ cells

$($ii$)7$ cells

$($iii$)12$ cells?

$[10$ marks$]$