Hi,

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 (guessing right) = .20 and q = .80

nCx =   |20C2 = 190

P(guessing 18 out of the 20 incorrectly) = 190(.20)^2(.80)^18 =.1369 

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