There was fruit whole sale packaging delivery company shipping fruits to various markets. Out of the items in a package there needs to be three boxes, one should have mango, another should have plums and one more should have mixure of both mango and plums. They are put inside three different boxes accordingly.
Just before packaging everything together in to one supervisor finds out that all the boxes have been labelled wrongly. Supersvisor opens one from the three boxes, and checks for the item inside. Immediately after that without opening rest of the boxes supervisor labels all the boxes correctly. Which box did the supervisor open and how did he/she know how to label all correct?