Question 621420
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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* {{{p^x*q^(n-x)}}} where p and q are the probabilities of success and failure respectively. 
In this case p & q are = 1/2 or .5
nCx = {{{n!/(x!(n-x)!)}}}
P(x&#8805; 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&#8805; 8) = P(8) + P(9) + P(10) = .055