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Nov 2019 p61 q2
3073
Annan has designed a new logo for a sportswear company. A survey of a large number of customers found that 42% of customers rated the logo as good.
On another occasion, a random sample of n customers of the company is chosen. Find the smallest value of n for which the probability that at least one person rates the logo as good is greater than 0.995.
Solution
The probability that a customer rates the logo as good is 0.42. Therefore, the probability that a customer does not rate the logo as good is 0.58.
The probability that none of the n customers rate the logo as good is given by:
\((0.58)^n\)
We want the probability that at least one person rates the logo as good to be greater than 0.995. Therefore, we have:
\(1 - (0.58)^n > 0.995\)
Rearranging gives:
\((0.58)^n < 0.005\)
Taking logarithms on both sides:
\(n \log(0.58) < \log(0.005)\)
Solving for n gives:
\(n > \frac{\log(0.005)}{\log(0.58)}\)
Calculating the right-hand side:
\(n > 9.727\)
Since n must be an integer, the smallest value of n is 10.