2.19: 4-digit codes from digits 1–9
A 4-digit code is to be formed using 4 different numbers selected from 1, 2, 3, 4, 5, 6, 7, 8 and 9.
(a)[1]
Find how many different codes can be formed if there are no restrictions.
(b)[1]
Find how many different codes can be formed if only prime numbers are used.
(c)[2]
Find how many different codes can be formed if two even numbers are followed by two odd numbers.
(d)[2]
Find how many different codes can be formed if the code forms an even number.