A die is biased so that the probability of throwing a 5 is 0.75 and the probabilities of throwing a 1, 2, 3, 4 or 6 are all equal.
Find the probability that, out of 10 throws of this die, at least 8 throws result in a 5.
Solution
The probability of throwing a 5 is given as 0.75. Therefore, the probability of not throwing a 5 is:
\(1 - 0.75 = 0.25\)
We need to find the probability of getting at least 8 fives in 10 throws. This is a binomial probability problem where:
\(n = 10, \ p = 0.75\)
The probability of getting exactly \(r\) fives is given by the binomial formula:
\(P(X = r) = \binom{n}{r} p^r (1-p)^{n-r}\)
We need to calculate \(P(X \geq 8) = P(X = 8) + P(X = 9) + P(X = 10)\).
Calculating each term:
\(P(X = 8) = \binom{10}{8} (0.75)^8 (0.25)^2\)
\(P(X = 9) = \binom{10}{9} (0.75)^9 (0.25)^1\)
\(P(X = 10) = \binom{10}{10} (0.75)^{10}\)
Substituting the values:
\(P(X = 8) = 45 \times (0.75)^8 \times (0.25)^2\)
\(P(X = 9) = 10 \times (0.75)^9 \times (0.25)\)
\(P(X = 10) = 1 \times (0.75)^{10}\)
Summing these probabilities gives:
\(P(X \geq 8) = 0.526\)
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