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June 2015 p61 q4
2617
A survey is undertaken to investigate how many photos people take on a one-week holiday and also how many times they view past photos. For a randomly chosen person, the probability of taking fewer than 100 photos is \(x\). The probability that these people view past photos at least 3 times is 0.76. For those who take at least 100 photos, the probability that they view past photos fewer than 3 times is 0.90. This information is shown in the tree diagram. The probability that a randomly chosen person views past photos fewer than 3 times is 0.801.
(i) Find \(x\).
(ii) Given that a person views past photos at least 3 times, find the probability that this person takes at least 100 photos.
Solution
(i) We know the total probability of viewing photos fewer than 3 times is 0.801. This can be expressed as:
\((1-x) \times 0.9 + x \times 0.24 = 0.801\)
Solving for \(x\):
\(0.9 - 0.9x + 0.24x = 0.801\)
\(0.9 - 0.66x = 0.801\)
\(0.66x = 0.9 - 0.801\)
\(0.66x = 0.099\)
\(x = \frac{0.099}{0.66} = 0.15\)
(ii) We need to find \(P(\text{at least 100 photos} \mid \text{at least 3 views})\).
Using Bayes' theorem:
\(P(\text{at least 100 photos} \mid \text{at least 3 views}) = \frac{P(\text{at least 100 photos} \cap \text{at least 3 views})}{P(\text{at least 3 views})}\)
\(P(\text{at least 100 photos} \cap \text{at least 3 views}) = 0.85 \times 0.1\)
\(P(\text{at least 3 views}) = 1 - 0.801 = 0.199\)
\(P(\text{at least 100 photos} \mid \text{at least 3 views}) = \frac{0.85 \times 0.1}{0.85 \times 0.1 + 0.15 \times 0.76}\)