Answer:
540x+680y<=20000
Explanation:
x will be the number of laptops. Since they are $540 each, 540x is the amount spent on laptops, depending on how many (x) are purchased.
y will be the number if desktops. Since they are $680 each, 680y is the amount spent on desktops, depending on how many (y) are purchased.
Adding all the money spent on laptops and desktops, we get
540x + 680y. This amount must be less than the $20000. It can be exactly $20000 also. But it cannot be more than $20000.
540x+680y<=20000