Question 394924
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Hi
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 = .9 and q = .1
nCx = {{{n!/(x!(n-x!))}}}

P(exactly 4 out of 7 time) = 35 (.9)^4(.1)^3 = .0229635