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June 2004 p6 q7
3058
A shop sells old video tapes, of which 1 in 5 on average are known to be damaged.
A random sample of 15 tapes is taken. Find the probability that at most 2 are damaged.
Solution
Let the random variable \(X\) represent the number of damaged tapes in a sample of 15. The probability of a tape being damaged is \(p = 0.2\), and the probability of a tape not being damaged is \(1 - p = 0.8\).
We use the binomial probability formula \(P(X = k) = \binom{n}{k} p^k (1-p)^{n-k}\) where \(n = 15\).