If 6Pr = 360 and If 6Cr = 15, find r ?  Choices:
A) 5 B) 6 C) 4 D) 3 E) 2

Rationale: solution : npr = ncr � r ! 6 pr = 15 * r ! 360 = 15 * r ! r ! = 360 / 15 = 24 r ! = 4 * 3 * 2 * 1 = > r ! = 4 ! therefore , r = 4 . answer : option c
Answer: C