Exercise 6. The words of the language ababaa are sequences of characters "a" or "b" such that every letter "b" is followed by an "a". Determine the number of 6-letter words in the language ababaa.
Solution of the exercise $6 \mathrm{Il}$ there is 1 word with no letter $b$, 5 words with one letter $b$ (in position $1,2,3,4$ or 5), 6 words with two letters $b$, 1 word with three letters $b$ so in total: 13 words.