He then went to see the oranges being harvested. He found out that they harvest 13 sacks per day. How many days will it take to harvest 13 sacks of oranges?
There are 13 sacks of oranges. 
Each day, they harvest the same number of sacks. 

How many days will it take to harvest all 13 sacks of oranges? 

It will take them 13 days to harvest all 13 sacks of oranges.