Hi,

 true-or-false questions... student knows the correct answer to six of them

 P(scoring ≥ 90%) = P(getting at least 3 of the remaining 4 correct)

 P(x≥3) = P(3) + P(4) = 4C3(.5)^3(.5)^1 + 4C4(.5)^4 



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 =  

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