1. The problem statement, all variables and given/known data From this combination of letters AAAXYZNO Find how many ways to pick 3 letters if the order does not matter. 3. The attempt at a solution I tried to elaborate it like this: We have ___ 3 empty spaces. A__ (Two empty space for other different letters) -> 5C2 = 10 AA_ (One empty space) -> 5C1 = 5 AAA (All AAA) -> 1 way only. ___ (No A) -> 5C3 = 10 The consideration is __A and A__ will be just the same because the order does not matter. Hence, total ways = 26. Or another way is simply 6C3 because we eliminate all of the As giving 20 ways only. I also want to ask if you guys know the insight that can be shared in solving this kind of problem since I feel the concept is just floating in my mind without concrete standing.