A person should choose 3 letters from a set containing the letters {a,b,c,d,e}

a) How many ways can this be done if repetition is allowed.

b) How many ways can this be done if repetition is not allowed.

c) How many ways can this be done if first letter must be a vowel and repetition is allowed.

d) How many ways can this be done if the first letter must be a constant and repetition is not allowed.