Feb/Mar 2020 p52 q2
3104
An ordinary fair die is thrown repeatedly until a 1 or a 6 is obtained.
Find the probability that it takes at least 3 throws but no more than 5 throws to obtain a 1 or a 6.
Solution
The probability of obtaining a 1 or a 6 on a single throw of a fair die is \(\frac{2}{6} = \frac{1}{3}\).
The probability of not obtaining a 1 or a 6 on a single throw is \(\frac{4}{6} = \frac{2}{3}\).
We need to find the probability that it takes at least 3 throws but no more than 5 throws to obtain a 1 or a 6.
This can be calculated as:
\(\left( \frac{1}{3} \right) \left( \frac{2}{3} \right)^2 + \left( \frac{1}{3} \right) \left( \frac{2}{3} \right)^3 + \left( \frac{1}{3} \right) \left( \frac{2}{3} \right)^4\)
Calculating each term:
\(\left( \frac{1}{3} \right) \left( \frac{2}{3} \right)^2 = \frac{4}{27}\)
\(\left( \frac{1}{3} \right) \left( \frac{2}{3} \right)^3 = \frac{8}{81}\)
\(\left( \frac{1}{3} \right) \left( \frac{2}{3} \right)^4 = \frac{16}{243}\)
Adding these probabilities:
\(\frac{4}{27} + \frac{8}{81} + \frac{16}{243} = \frac{76}{243}\)
Thus, the probability is \(\frac{76}{243}\) or approximately 0.313.
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