June 2013 p63 q4
3076
In a certain country, on average one student in five has blue eyes.
For a random selection of n students, the probability that none of the students has blue eyes is less than 0.001. Find the least possible value of n.
Solution
The probability that a student does not have blue eyes is given by:
\(P( ext{not blue eyes}) = 1 - \frac{1}{5} = \frac{4}{5} = 0.8\)
For n students, the probability that none of them has blue eyes is:
\((0.8)^n\)
We need this probability to be less than 0.001:
\((0.8)^n < 0.001\)
Taking logarithms on both sides:
\(n \log(0.8) < \log(0.001)\)
Solving for n:
\(n > \frac{\log(0.001)}{\log(0.8)}\)
Calculating the values:
\(n > \frac{-3}{-0.09691} \approx 30.9\)
Since n must be an integer, the least possible value of n is 31.
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