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June 2010 p63 q3
3071
Christa takes her dog for a walk every day. The probability that they go to the park on any day is 0.6. If they go to the park there is a probability of 0.35 that the dog will bark. If they do not go to the park there is a probability of 0.75 that the dog will bark.
(i) Find the probability that they go to the park on more than 5 of the next 7 days.
(ii) Find the variance of the number of times they go to the park in 30 days.
Solution
(i) We use the binomial distribution where the probability of going to the park on any day is 0.6. We need to find the probability of going to the park more than 5 times in 7 days, i.e., 6 or 7 times.
The probability of going exactly 6 times is given by: