June 2011 p63 q6
3079
The probability that Sue completes a Sudoku puzzle correctly is 0.75.
Sue attempts n Sudoku puzzles. Find the least value of n for which the probability that she completes all n puzzles correctly is less than 0.06.
Solution
The probability that Sue completes all n puzzles correctly is given by \((0.75)^n\).
We need to find the smallest n such that \((0.75)^n < 0.06\).
Taking logarithms on both sides, we have:
\(\log((0.75)^n) < \log(0.06)\)
\(n \cdot \log(0.75) < \log(0.06)\)
\(n > \frac{\log(0.06)}{\log(0.75)}\)
Calculating the right-hand side gives \(n > 9.78\).
Since n must be an integer, the smallest possible value is \(n = 10\).
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