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Hi,
Suppose you are asked to pick '3 numbers' from 1 to 20 to win a prize
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(picking the winning number)= 1/20 and q = 19/20
nCx = 
P(picking the winning number) = 3C1(.05)^1 (.95)^2 = .1354