Chapter 11 - Further Topics in Algebra - 11.6 Basics of Counting Theory - 11.6 Exercises - Page 1060: 3

$6$

In a number, order of the digits is important. Permutations. We are taking r=3 from n=3 digits, $P(n,r)=\displaystyle \frac{n!}{(n-r)!}$. $ P(3,3)=\displaystyle \frac{3!}{(3-3)!}\qquad$ ... 0!=1, by definition $P(3,3)=3!=3\times 2\times 1=6$