{all words with A, B, or C}   3^4
- (
    {words without A}         2^4
  - {words without B}         2^4
  + {words without A or B}    1^4
)

= 3^4 - (2^4 + 2^4 - 1^4)
= 3^4 - 2^4 - 2^4 + 1^4
= 81 - 16 - 16 + 1
= 50