中等MATH-PROB-007
A password is an ordered pair of distinct letters chosen from A, B, C, D and E. How many passwords are there?
A. 10
B. 20✓ 答案
C. 25
D. 32
解說
Order matters and letters are not repeated, so the count is P(5, 2) = 5 × 4 = 20. The value 10 is C(5, 2). The value 25 allows repeated letters.