It depends how many hrs it take for him to mow a lawn. For example, if he worked 3 hours on a lawn he would get 27.75 or 9.25 times 3. Minus 7.50 = 20.50
If you take the 2500$ he makes base salary and subtract from 4000$ you get 1500$ and if you multiply it by 1.80 you get the other needed 80% of the required sales.