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June 2015 p63 q1
3234
The weights, in grams, of onions in a supermarket have a normal distribution with mean \(\mu\) and standard deviation 22. The probability that a randomly chosen onion weighs more than 195 grams is 0.128. Find the value of \(\mu\).
Solution
Given that the probability \(P(X > 195) = 0.128\), we need to find the mean \(\mu\) of the normal distribution.
First, find the z-score corresponding to the probability 0.128. From standard normal distribution tables, \(P(Z > 1.136) = 0.128\), so \(z = 1.136\).
Using the standardization formula:
\(z = \frac{X - \mu}{\sigma}\)
Substitute the known values:
\(1.136 = \frac{195 - \mu}{22}\)
Solving for \(\mu\):
\(1.136 \times 22 = 195 - \mu\)
\(24.992 = 195 - \mu\)
\(\mu = 195 - 24.992\)
\(\mu = 170.008\)
Rounding to the nearest whole number, \(\mu = 170\).