How many strings of eight english letters are there a. that contain no vowels, if letters can be repeated? b. that contain no vowels, if letters cannot be repeated? c. that start with a vowel, if letters can be repeated? d. that start with a vowel, if letters cannot be repeated? e. that contain at least one vowel, if letters can be repeated? f ) that contain exactly one vowel, if letters can be repeated? g) that start with x and contain at least one vowel, if letters can be repeated? h) that start and end with x and contain at least one vowel, if letters can be repeated?