Feb/Mar 2018 p62 q8
3301
The results of a survey at a certain large college show that the proportion of students who own a car is \(\frac{1}{4}\).
For a random sample of 160 students at the college, use a suitable approximate distribution to find the probability that fewer than 50 own a car.
Solution
Let \(p = 0.25\) and \(n = 160\). The mean of the distribution is \(n \times p = 160 \times 0.25 = 40\).
The variance is \(n \times p \times (1-p) = 160 \times 0.25 \times 0.75 = 30\).
We approximate the binomial distribution with a normal distribution: \(X \sim N(40, 30)\).
To find \(P(X < 50)\), we use the continuity correction: \(P(X < 49.5)\).
Standardize: \(Z = \frac{49.5 - 40}{\sqrt{30}} \approx 1.734\).
Thus, \(P(Z < 1.734) = 0.959\).
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