total number of digits= 10 (from 0 to 9)
total number of letters = 6 (from A to F)
probability of numbers = 10/(10+6) = 0.625
this is a case of binomial distribution with fixed number trials n = 32 and probability p = 0.625
we have to find probability of at least 20 numbers
P(at least 20 numbers) = 1 - BINOMDIST(numbers, trials, probability,true)
setting numbers = 20-1, trials = 32 and probability = 0.625
we get
(BINOMDIST is excel function)
P(X > 20) = 1 - BINOMDIST(20 - 1,32,0.625, true = 1-0.4219 = 0.578
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