With 26 alphabet for a 5 letter word we can have $26^{5}$ possible words as each 5 position can have 26 possible alphabet.
we need no of word which contains at least one ‘A’.
This is same as
Total number of words – Number of words with no ‘A’
Now,
Number of words with no ‘A’ =$25^{5}$ as, each 5 position can have 25 possible alphabet as we exclude ‘A’.
so ,
Total number of words which contains at least one ‘A’ =$26^{5}-25^{5}$.