Chapter 11 - Further Topics in Algebra - 11.6 Basics of Counting Theory - 11.6 Exercises - Page 1061: 15

$132$

$P(n,r)=\displaystyle \frac{n!}{(n-r)!}$. Factorials: $n!=n(n-1)!=n(n-1)(n-2)!=...\\n!=n(n-1)(n-2)\cdots(3)(2)(1)$ $ 0!=1,\qquad$ by definition. --- $P(12,2)=\displaystyle \frac{12!}{(12-2)!}=\frac{12\times 11\times 10!}{10!}$ $=12\times 11=132$