Answer: 248 boxes
Explanation:
They produce boxes at a rate of 52 boxes an hour yet after only 3 hours they had 404 boxes.
The number they were supposed to have in 3 hours is:
= 52 * 3
= 156 boxes
They instead had 404 so the rest must have been there when they opened:
= 404 - 156
= 248 boxes