Are there efficient methods of findings the answer to combination problems like these? The letters A, A, B and C are arranged in any order. How many DISTINCT sequences can we form? If two letters weren't the same, then it would be simple. I can find the answer relatively easily since it's only 4 letters. Imagine if this was a sequence of 10 letters where two letters were the same. Then creating all possible combinations would be really inefficient and laboring. Is this any trick for problems where duplicate letters exist? P.S. By the way the answer is 12. I found it by creating a tree diagram.