Solve it by enumeration method. All the possible alternatives are given in the table below. Calculate each alternative's vehicle hours Calculate its objective if the solution is feasible. You may use Excel to do all the calculations automatically first. Then punch in the $n$ Answer formats: If a solution is infeasible, do not calculate its objective; just write $0.$ Do not put a dollar sign.

$\backslash$table$[[$Alterna$,$X$1,x2,$X$3,x4,$Vehicle hours needed,Objective$],[1,0,0,0,0,,],[2,1,0,0,0,,],[3,0,1,0,0,,],[4,0,0,1,0,,],[5,0,0,0,1,,],[6,1,1,0,0,,],[7,1,0,1,0,,],[8,1,0,0,1,,],[9,0,1,1,0,,],[10,0,1,0,1,,],[11,0,0,1,1,,],[12,1,1,1,0,,],[13,1,1,0,1,,],[14,1,0,1,1,,],[15,0,1,1,1,,],[16,1,1,1,1,,]]$