Answer:
The librarian can buy a maximum of 20 chairs to stay within the budget.
Explanation:
The total budget is:
T = 800
From that budget, he spends 320 on desks (D), so he has the remaining amount to buy chairs (C):

Since each chair costs 24, and he has 480 he can buy the following number of chairs:

Therefore, the librarian can buy a maximum of 20 chairs to stay within the budget.
I hope it helps you!