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Nov 2012 p62 q4
3322
The mean of a certain normally distributed variable is four times the standard deviation. The probability that a randomly chosen value is greater than 5 is 0.15.
200 values of the variable are chosen at random. Find the probability that at least 160 of these values are less than 5.
Solution
Let the probability that a value is less than 5 be denoted by \(p\). Since the probability that a value is greater than 5 is 0.15, we have \(p = 0.85\).
The mean number of values less than 5 in 200 trials is \(\mu = 200 \times 0.85 = 170\).
The variance is \(\text{var} = 200 \times 0.85 \times 0.15 = 25.5\).
We need to find \(P(\text{at least 160})\), which is equivalent to \(P(X \geq 160)\).
Using continuity correction, this becomes \(P(X > 159.5)\).