June 2016 p62 q6
3141
The time in minutes taken by Peter to walk to the shop and buy a newspaper is normally distributed with mean 9.5 and standard deviation 1.3.
- Find the probability that on a randomly chosen day Peter takes longer than 10.2 minutes.
- Calculate an estimate of the number of days in a year (365 days) on which Peter takes less than 8.8 minutes to walk to the shop and buy a newspaper.
Solution
(i) To find the probability that Peter takes longer than 10.2 minutes, we standardize the variable:
\(P(x > 10.2) = P\left( z > \frac{10.2 - 9.5}{1.3} \right)\)
\(= P(z > 0.53846)\)
Using the standard normal distribution table, \(P(z > 0.53846) = 1 - 0.7046 = 0.295\).
(ii) To find the number of days Peter takes less than 8.8 minutes, we use symmetry:
\(P(x < 8.8) = 0.2954\)
Estimate the number of days: \(365 \times 0.2954 = 107.671\)
Rounding gives approximately 107 or 108 days.
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