How can I get the total number of 7-character format license plates (capital letter or digit) if:
a) There are no further restrictions:
Number of Capital letters = 26 (A-Z)
Number of Digits = 10 (0-9)
36×35×34×33×32×31×30 = 42072307200
b) If the first 3 characters are letters and the last 4 are numbers:
26×25×24×10×9×8×7 = 78624000
c) If letters and numbers alternate, for example A3B5A7Q or 0Z3Q4Q9:
Number of permutations that start with a number:
10×26×9×25×8×24×7 = 78624000
Number of permutations that start with a letter:
26×10×25×9×24×8×23 = 258336000
Total number of permutations:
78624000 + 258336000 = 336960000
What can be wrong with this approach?