Feb/Mar 2019 p62 q3
3206
The times taken, in minutes, for trains to travel between Alphaton and Beeton are normally distributed with mean 140 and standard deviation 12.
The probability that a randomly chosen train takes more than k minutes to travel between Alphaton and Beeton is 0.675. Find the value of k.
Solution
Given that the probability that the time is greater than k is 0.675, we have:
\(P(X > k) = 0.675\)
Using the standard normal distribution, we find the corresponding z-value for the cumulative probability of \(1 - 0.675 = 0.325\).
The z-value for 0.325 is approximately \(z = -0.454\).
Using the standardization formula:
\(z = \frac{k - 140}{12}\)
Substitute \(z = -0.454\):
\(\frac{k - 140}{12} = -0.454\)
Solving for k:
\(k - 140 = -0.454 \times 12\)
\(k - 140 = -5.448\)
\(k = 140 - 5.448\)
\(k = 134.552\)
Rounding to two decimal places, \(k = 134.55\).
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