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Nov 2012 p61 q3
3255
Lengths of rolls of parcel tape have a normal distribution with mean 75 m, and 15% of the rolls have lengths less than 73 m.
Find the standard deviation of the lengths.
Alison buys 8 rolls of parcel tape.
Find the probability that fewer than 3 of these rolls have lengths more than 77 m.
Solution
(i) To find the standard deviation \(\sigma\), we use the z-score formula:
\(z = \frac{X - \mu}{\sigma}\)
Given \(X = 73\), \(\mu = 75\), and \(z = -1.036\) (from the z-table for 15%), we have:
\(-1.036 = \frac{73 - 75}{\sigma}\)
Solving for \(\sigma\):
\(\sigma = \frac{2}{1.036} \approx 1.93\)
(ii) The probability that a roll is longer than 77 m is 0.15. We need to find the probability that fewer than 3 out of 8 rolls are longer than 77 m. This is a binomial probability problem with \(n = 8\) and \(p = 0.15\).