$4-28.$ In Example $4\mathrm{.}3-1,$ compute the change in the optimal revenue in each of the following

cases $($use TORA output to obtain the feasibility ranges$)$:

$($a$)$ The constraint for raw material M$1($resource $1)$ is $6$x$1+4$x$2...20.$

$($b$)$ The constraint for raw material M$2($resource $2)$ is x$1+2$x$2...5.$

$($c$)$ The market condition represented by resource $4$ is x$2...4.$