For example we have a box with five balls : 4 white balls and one black. Every trial we take on ball and then put it back. How do we determine probability of taking black ball 2 times of 10 trials?
The experiment which has two outcomes "success" (taking black ball) or "failure" (taking white one) is called Bernoulli trial. The experiment with a fixed number n of Bernoulli trials each with probability p, which produces k success outcomes is called binomial experiment.
Probability of k successes in n Bernoulli trials is given as:
where p - is a probability of each success event, - Binomial coefficient or number of combinations k from n
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Probability of taking black ball in k first trials of n total trials is given as:
it's a probability of only one possible combinations. According to combinatorics formulas the following k success combinations number is possible in n trials: see Combinatorics. Combinations, arrangements and permutations.
Number of success events k in n statistically independent binomial trials is a random value with the binomial distribution, see: Binomial distribution, probability density function, cumulative distribution function, mean and variance