to find # of ways to have exactly 3 digits 6 i used the formula 9*9*5P5/3! ( the order is important, 12 not equal 21, we also need to divide by 3!, because we have 3 indistinguishable numbers ( 3 digits of 6). The solutions is 9*9*5C3. Could you hep me to understand what my mistake is.
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