He then went to see the oranges being harvested. He found out that they harvest w sacks per day. How many days will it take to harvest x sacks of oranges?
Let's say that w sacks per day is the rate of harvesting. The number of days it will take to harvest x sacks of oranges is denoted by d. 

d = x/w

This equation states that the number of days it will take to harvest x sacks of oranges is equal to x sacks divided by w sacks per day.