Suppose the 5 letters are A, B, C, D and E.
The letter A can either be in the combination or not: 2 options for A.
With each of these options, B can either be in the combination or not: 2 options for B - making 2*2 options so far.
With each of the options so far, C can either be in the combination or not: 2 options for C - making 2*2*2 options so far.
and so on.
So for 5 letters there are 25 = 32 combinations. However, one of these is the combination that excludes each of the 5 letters - ie the null combination. Excluding the null combination gives the final answer of 31 combinations.
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120 ABCDE , there are 120 ways these five letters can be rearranged.
5C3 = 10
one hundred and twenty
3 x 5 = 15
Q: How many letters of alphabet can you make using 5 squares?