Consider Exercise 1 of Chapter 2, Section 1 on allocation of city funds for the purchase of two types of vehicles. Suppose that the purchase price of vans is incremented by an amount \(\Delta_{1}\), and the maintenance cost per year for a van is changed by an amount \(\Delta_{2}\). Characterize the set of all such \(\Delta_{1}\) and

\(\Delta_{2}\), such that the old optimal solution computed in that problem is still optimal.