There isn't enough information because we don't know the amount Jill brought. We know the amounts Bill and Mike bought, but we know nothing about Jill.
2 1/3 + 1 3/4 = 2 4/12 + 1 9/12 = 3 13/12 = 4 1/12
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Answer: Bill and Mike bought 4 1/12 boxes of muffins.
