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?
Let's denote the number of sacks of oranges per day by x. Then the number of sacks of oranges per day that they discard is 71. 

The number of sacks of oranges that they have after 51 days of harvest is 

x*51-71*51=2379. 

Therefore the answer (numeric) is: 

2379