How to use Permutation/Combination to find max possible codons?

[dannys00]

I keep reading that 64 is the max possible number of codons with 4 letters to choose from (A,U,C,G). Watching a Khan academy video, he wrote that its 4x4x4 because there are 3 slots open for the 4 possible letters. How can this be put into a combination formula? I thought it would be 4!/ 3!(4!-3!) But that only gets an answer of 4. What other number should be in place of these to get the 64 possible codons?

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