Question 841541
(a)the numbers must be odd
<pre>
Always choose the most restrictive digit first.

Choose the last digit any of these: 1,3,5,7,9.  That's 5 ways.
Choose the first digit any of the 8 remaining ways.
Choose the second digit any of the 7 remaining ways.
Choose the third digit any of the 6 remaining ways.
Choose the fourth digit any of the 5 remaining ways.

Answer = 5򊤋򉼱 = 8400 

2. Solve the proceeding problem if repetitions of digits are allowed. 

Always choose the most restrictive digit first.

Choose the last digit any of these: 1,3,5,7,9.  That's 5 ways.
Choose the first digit any of the 9 ways.
Choose the second digit any of the 9 ways.
Choose the third digit any of the 9 ways.
Choose the fourth digit any of the 9 ways.

Answer = 5򊷹򊷹 = 32805

Edwin</pre>