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Feb/Mar 2016 p62 q5
3090
In a certain town, 35% of the people take a holiday abroad and 65% take a holiday in their own country. Of those going abroad 80% go to the seaside, 15% go camping and 5% take a city break. Of those taking a holiday in their own country, 20% go to the seaside and the rest are divided equally between camping and a city break.
A group of n people is chosen randomly. The probability of all the people in the group taking a holiday in their own country is less than 0.002. Find the smallest possible value of n.
Solution
The probability that a person takes a holiday in their own country is 0.65. For a group of n people, the probability that all take a holiday in their own country is given by:
\((0.65)^n < 0.002\)
Taking logarithms on both sides, we have:
\(n \cdot \log(0.65) < \log(0.002)\)
Solving for n, we get:
\(n > \frac{\log(0.002)}{\log(0.65)}\)
Calculating the right-hand side:
\(\log(0.002) \approx -2.69897\)
\(\log(0.65) \approx -0.18709\)
Thus:
\(n > \frac{-2.69897}{-0.18709} \approx 14.43\)
Since n must be an integer, the smallest possible value of n is 15.