$($a$)$ A departmental head has four subordinates and four tasks to be performed. The subordinates differ in efficiency and the tasks differ in difficulty. His estimate of the time each man would take to perform each task is given below. How should the tasks be allotted so as to minimize the total manhours?

$\backslash$table$[[,$Tasks,$],[,,1,2,3,4],[,,18,26,17,11],[,,13,28,14,26],[,$C$,38,19,18,15],[,,19,26,24,10]]$