In Example $4\mathrm{.}3-1,$ compute the change in the optimal revenue in each of the following cases assuming that they are within their respected feasibility ranges:

a$.$ The constraint for raw material $M1($resource $1)$ is $6x_1+4x_{2}20.$

b$.$ The constraint for raw material $M2($resource $2)$ is $x_1+2x_{2}5.$

c$.$ The market condition represented by resource $4$ is $x_{2}4.$