Write the Word or Phrase That Best Completes Each Statement ${ } _ { n } P _ { r }$

Short Answer

Write the word or phrase that best completes each statement or answers the question.
Provide the missing information.
-The number of permutations of n elements taken r at a time is denoted by ${ } _ { n } P _ { r }$ and is computed by ${ } _ { n } P _ { r } = \frac { \square } { \square } \text { or } { } _ { n } P _ { r } = n ( n - 1 ) ( n - 2 ) \ldots ( n - r + 1 )$