Problem of the week

In a PIN consisting of 5 characters, at least two special symbols (, , ) are used apart from the digits 0 to 9 due to security reasons.

        a) How many such PINs do exist?

    How many of these have

        b) only different special characters, e. g. 0 9 ,
        c) at least two identical special characters, e. g. 5 ,
        d) at least two identical digits, e. g. 33 3 ?

Answer:

a)
b)
c)
d)