How many strings of eight English letters are there: (Solve by giving formula used for each bit). a) that start with a vowel, if letters can be repeated? b) that

start with a vowel, if letters cannot be repeated? c) that contain at least one vowel, if letters can be repeated? d) that contain exactly one vowel, if letters can be repeated? e) that start with X and contain at least one vowel, if letters can be repeated?