Consider a profit maximization problem constrained by availability of labor and material that has been formulated as the linear program given below:
max[10x1 + 8x2]
s.t. x1 + 3x2 ≤ 30 (labor)
6x1 + 3x2 ≤ 120 (material)
x1 , x2 ≥ 0
Suppose the availability of labor changed from 30 to 30+?. Determine the range of values of ? for which the same variables remain in the optimal tableau?