csat-suneung 2005 Q14

csat-suneung · South-Korea · csat__math-humanities 4 marks Combinations & Selection Counting Arrangements with Run or Pattern Constraints

Among 12-character strings made using all eight $a$'s and four $b$'s, how many strings satisfy all of the following conditions? [4 points]
(a) $b$ cannot appear consecutively.
(b) If the first character is $b$, then the last character is $a$.
(1) 70
(2) 105
(3) 140
(4) 175
(5) 210

Among 12-character strings made using all eight $a$'s and four $b$'s, how many strings satisfy all of the following conditions? [4 points]\\
(a) $b$ cannot appear consecutively.\\
(b) If the first character is $b$, then the last character is $a$.\\
(1) 70\\
(2) 105\\
(3) 140\\
(4) 175\\
(5) 210

Paper Questions

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