He then went to see the oranges being harvested. He found out that they harvest w sacks per day and discard x of them. How many sacks of oranges will they have after y days of harvest?
First, we need to find out how many sacks of oranges they will have after one day of harvest. They harvest w sacks per day and discard x of them, so they will have w-x sacks of oranges after one day.

Next, we need to find out how many sacks of oranges they will have after two days of harvest. They will have w-x sacks of oranges after the first day, and they will harvest w sacks per day and discard x of them on the second day, so they will have (w-x) + (w-x) = 2(w-x) sacks of oranges after two days.

We can continue this pattern to find out how many sacks of oranges they will have after y days of harvest. They will have (w-x) + (w-x) + ... + (w-x) = y(w-x) sacks of oranges after y days.