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?
Let's say that on the first day they harvest w1 sacks of oranges and discard x1 of them. Then they have w1-x1 sacks of oranges. On the second day, they harvest w2 sacks of oranges and discard x2 of them. They now have (w1-x1)+(w2-x2) sacks of oranges. 

Can you see the pattern? On the third day, they will have (w1-x1)+(w2-x2)+(w3-x3) sacks of oranges. 

In general, on the yth day, they will have (w1-x1)+(w2-x2)+...+(wy-xy) sacks of oranges.