A bakery sells three kinds of rolls. How many different combinations of rolls could Jack purchase if he buys a total of six rolls and includes at least one of each kind?
Let's call the rolls roll A, B, and C We can just multiply all combinations by 3 and get the answer because AAAABC would just be BBBBAC and CCCCBA following the same pattern ABCABC
AAAABC * 3 = 3
AAABCC * 3 = 9 AAABBC
AABBCC *3 = 18 AABCCC AABBBC
ABBBCC * 3 = 30 ABBCCC ABCCCC ACBBBB
Curiously enough to went 3 + 6 + 9 + 12. Answer is 30
