Well Darius could of had 6 friends, each got 3 muffins. 3 friends each got 6 muffins, 2 friends each got 9 muffins, or 9 friends each got 2 muffins. Now he could of had 1 friend and they got all the muffins but in the question there's a "s" on "friends". What I simply down was got all the multiples of 18 and made them fit with the numbers being either friends or muffins.
