Answer:
For the number of lawns that he mows:
Earning = 20x
For the number of gardens that he weeds:
Earning = 15y
Explanation:
Since you do not know how much he will earn you will put x and y and the 20 and 15 are the cost of one so then you just multiply and if you are given the number of times he mows then you just add them up together to get the earning of $4000