Step-by-step explanation:
nnnllll
the first number has 10 options (0 .. 9).
the second number has 9 options, as repeats are not allowed.
for the same reasons the third number has then only 8 options.
then, the first letter has 26 options (a .. z).
the second letter has 25 options. again, because repeats are not allowed.
the third letter has then 24 options.
the fourth letter only 23 options.
so, altogether, for the whole password, we have
10×9×8×26×25×24×23 possible passwords.
that are
258,336,000
possible passwords
