1 .
Consider strings of length 15, where each character is a lowercase letter or an uppercase letter.
How many such strings contain at least one lowercase letter and at least one uppercase letter?
(a)
None of the above.
(b)
$52^{15} - 2 \cdot 26^{15}$
(c)
$52^{15} - 26^{15}$
(d)
$52^{15} - 3 \cdot 26^{15}$