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Why can't I use a "probability way":

Four correct: (1/3)^4 x (2/3) = 2/243

Five correct: (1/3)^5 = 1/243

2/243 + 1/243 = 3/243 (which is obv wrong but I don't understand why).

Bunuel
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Lodz697

A multiple choice examination has 5 questions. Each question has three alternative answers of which exactly one is correct. The probability that a student will get 4 or more answers correct just by guessing is

A) 17/(3^5)
B) 13/(3^5)
C) 11/(3^5)
D) 10/(3^5)
E) 12/(3^5)


Why can't I use a "probability way":

Four correct: (1/3)^4 x (2/3) = 2/243

Five correct: (1/3)^5 = 1/243

2/243 + 1/243 = 3/243 (which is obv wrong but I don't understand why).

Bunuel
It should be:

P(4 correct) + P(5 correct) =

= (1/3)^4 *2/3*5!/4! +  (1/3)^5 = 

= 11/243.

We multiply (1/3)^4 *2/3 by 5!/4! = 5 becasue CCCCW can be obtained in different ways: CCCCW, CCCWC, CCWCC, CWCCC, and WCCCC.

Answer: C.
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