He then went to see the oranges being harvested. He found out that they harvest 74 sacks per day and discard 71 of them. How many sacks of oranges will they have after 51 days of harvest?
On the first day, they harvest 74 sacks and discard 71 of them. They are left with 3 sacks. 

On the second day, they harvest 74 sacks and discard 71 of them. They are left with 6 sacks (3 from the first day, plus 3 from the second day). 

On the third day, they harvest 74 sacks and discard 71 of them. They are left with 9 sacks (6 from the first two days, plus 3 from the third day). 

This pattern continues. After the 51st day, they will have 3*51=153 sacks of oranges.