June 2015 p61 q6
3315
(ii) Use an approximation to find the probability that, in a random sample of 500 households, more than 337 households have a printer.
(iii) Justify your use of the approximation in part (ii).
Solution
(ii) To approximate the probability, we use the normal approximation to the binomial distribution. Given that the sample size is 500, we calculate:
Mean: \(np = 340\)
Variance: \(npq = 108.8\)
We standardize the variable using continuity correction:
\(P(x > 337) = P\left( z > \frac{337.5 - 340}{\sqrt{108.8}} \right)\)
\(= P(z > -0.2396)\)
Using the standard normal distribution table, we find:
\(P(z > -0.2396) = 0.595\)
(iii) The normal approximation is justified because both \(np = 340\) and \(nq = 160\) are greater than 5.
Log in to record attempts.