For example, “abfde” CANNOT be a password. Passwords with repeated letters are allowed.
(a) If the length of the password is 2, how many different passwords may be formed?
(b) If the length of the password is 3, find the number of different passwords begins with ‘a’ or ends with ‘e’ may be formed
The letters of a password may be chosen from only one group of the two groups of letters (a, b, c, d, e) and (f, g, h, i, j)?
2017-09-19 12:30 am
回答 (1)
2017-09-19 4:28 pm
✔ 最佳答案
(a)n( group (a, b, c, d, e) ) + n( group (f, g, h, i, j) )
= 5² + 5²
= 50 ..... Ans
(b)
n( begins with ‘a’ and ends with 'e' ) = 1 * 5 * 1 = 5
n( begins with ‘a’ and ends not 'e' ) = 1 * 5 * 4 = 20
n( begins not ‘a’ and ends with 'e' ) = 4 * 5 * 1 = 20
5 + 20 + 20
= 45 ..... Ans
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