He then went to see the oranges being harvested. He found out that they harvest 13 sacks per day and discard 3 of them. How many oranges do they harvest per day if each sack contains 38 oranges?
1. He sees 13 sacks being harvested per day. 
2. He sees that 3 of those sacks are discarded. 
3. We want to know how many oranges are harvested per day. 

We can set up a proportion to solve this problem: 

$$\frac{13}{38}=\frac{x}{38-3}$$

Cross multiply to solve for x: 

$$13(38-3)=x(38)$$

$$13(35)=x(38)$$

$$455=x(38)$$

$$12.1=x$$

So, they harvest 12.1 sacks of oranges per day.