for a school outing, the school has 20 small ice chests, each of which can hold up to six cans of soda. at the beginning of the day, 10 of the ice chests are full, with six cans each, and ten are empty. the teachers are going to rearrange where the cans of soda are stored. if every can of soda must be in an ice chest, and each ice chest must have at least one can and not more than six, then what is the largest number of ice chests that can have exactly four cans?
