Hi,

A student randomly guesses on a 10 question true/false exam. Minimum threshold for a B grade is 80%.  

Find the probablility the student gets at least a B on the exam.

Note: The probability of x successes in n trials is: 

P = nCx*  where p and q are the probabilities of success and failure respectively. 

In this case p & q are = 1/2 or .5

nCx = 

P(x≥ 8) = P(8) + P(9) + P(10)  

P(8) = 10C8(.5)^8(.5)^2  = 45(.5)^8(.5)^2 = .044

P(9) = 10C9(.5)^9(.5)^1 =10(.5)^9(.5)^1 = .01

P(10) = 10C10(.5)^10(.5)^0 =1*(.5)^10(.5)^0 = .001

P(x≥ 8) = P(8) + P(9) + P(10) = .055 

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