Chapter 1, Equations and Inequalities - 1.2 Evaluate and Simplify Algebraic Expressions - 1.2 Exercises - Problem Solving - Page 15: 62

See below.

Let $x$ be the number of desktop computers bought ($0\leq x\leq80$). Then the number of laptops bought is: $80-x$. Then the amount spent if we buy $x$ desktop computers is: $f(x)=x\cdot900+(80-x)\cdot1550=900x+124000-1550x=124000-650x$. Thus if $65$ choose desktops, the cost is: $f(65)=124000-650\cdot65=81750$