$6\mathrm{.}2..$ Consider the transportation problem having the unit shipping costs shown in the following table.

$\backslash$table$[[,$Destination,$],[,1,2,3,$Supply$],[$Source$,6,7,4,40],[1,5,8,6,60],[2,30,40,30,],[$Demand$,,,,]]$

a$.$ Formulate a network model for this problem as a minimum$-$cost flow problem by drawing a network similar to Figure $6\mathrm{.}3.$

b$.$ Formulate and solve a spreadsheet model for this problem.