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June 2017 p61 q5
3087
Eggs are sold in boxes of 20. Cracked eggs occur independently and the mean number of cracked eggs in a box is 1.4.
Calculate the probability that a randomly chosen box contains exactly 2 cracked eggs.
Calculate the probability that a randomly chosen box contains at least 1 cracked egg.
A shop sells n of these boxes of eggs. Find the smallest value of n such that the probability of there being at least 1 cracked egg in each box sold is less than 0.01.
Solution
(i) The mean number of cracked eggs is 1.4, so the probability of a cracked egg, p, is given by:
\(p = \frac{1.4}{20} = 0.07\)
The probability of exactly 2 cracked eggs in a box of 20 is calculated using the binomial distribution:
\(P(2) = \binom{20}{2} (0.07)^2 (0.93)^{18}\)
Calculating this gives:
\(P(2) = 0.252\)
(ii) The probability of at least 1 cracked egg is: