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June 2010 p61 q5
3049
In the holidays Martin spends 25% of the day playing computer games. Martin’s friend phones him once a day at a randomly chosen time.
Find the probability that, in one holiday period of 8 days, there are exactly 2 days on which Martin is playing computer games when his friend phones.
Solution
This problem can be modeled using a binomial distribution where the probability of success (Martin playing games when his friend phones) is 0.25, and the probability of failure is 0.75. We are looking for the probability of exactly 2 successes in 8 trials.
The probability mass function for a binomial distribution is given by:
\(P(X = k) = \binom{n}{k} p^k (1-p)^{n-k}\)
where \(n\) is the number of trials, \(k\) is the number of successes, and \(p\) is the probability of success.