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June 2004 p6 q7
3347
A shop sells old video tapes, of which 1 in 5 on average are known to be damaged.
A random sample of 1600 tapes is taken. Use a suitable approximation to find the probability that there are at least 290 damaged tapes.
Solution
Let the random variable \(X\) represent the number of damaged tapes in a sample of 1600 tapes. Given that 1 in 5 tapes are damaged, the probability of a tape being damaged is \(p = 0.2\).
The expected number of damaged tapes is \(\mu = np = 1600 \times 0.2 = 320\).
The variance is \(\sigma^2 = np(1-p) = 1600 \times 0.2 \times 0.8 = 256\).
We approximate the binomial distribution with a normal distribution: \(X \sim N(320, 256)\).
We need to find \(P(X \geq 290)\). Using continuity correction, this is \(P(X > 289.5)\).