Exercise 3.

We construct 4-digit numbers, which are chosen from any of the following seven digits: 0, 1, 3, 5, 7, 8, 9.

Answer the following:

a) How many different numbers are possible if a digit can be used more than once?

b) How many different numbers are there that contain no identical digits?

c) What is the probability of a number having no digit '7' and containing two or more identical digits?