Answer:
4 packs
Explanation:
There were 56 juice boxes total. They had 29 remaining, so we do 56-29 to get 27. When they open 4 packs, they would have had 28 but since they only had 27, there was 1 left in the 4th pack. This means that they still had to open the 4th pack, even though they didn't finish the whole pack
Hope this helps