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?
First, we need to find out how many sacks of oranges they will have after one day of harvest. They harvest 74 sacks per day and discard 71 of them, so they will have 3 sacks of oranges after one day of harvest. 

Next, we need to find out how many sacks of oranges they will have after 51 days of harvest. They will have 3 sacks of oranges after one day of harvest, so they will have 153 sacks of oranges after 51 days of harvest. 

Therefore the answer (numeric) is: 

153