Permutations and Counting problem? > Postal codes in Canada have six characters with alternating letters and digits in the form L#L#L#. How many postal codes do not have one letter repeated three times? What I did is...--prophetes.ai

Permutations and Counting problem? > Postal codes in Canada have six characters with alternating letters and digits in the form L#L#L#. How many postal codes do not have one letter repeated three times? What I did is $261025102410=15600000$, which is apparently wrong. The solution in the marking scheme is like this: Total number of codes: $ 261026102610=17576000$ Number of codes with the same letter 3 times is: $261010*10=26000$ Number of codes without the repeated letters is: $17576000-26000=1755000$ What is wrong with my logic? And I don't get the method the marking scheme used.

Your calculation also does not allow a letter to be repeated even twice.