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June 2015 p62 q4
2616
Nikita goes shopping to buy a birthday present for her mother. She buys either a scarf, with probability 0.3, or a handbag. The probability that her mother will like the choice of scarf is 0.72. The probability that her mother will like the choice of handbag is x. This information is shown on the tree diagram. The probability that Nikita’s mother likes the present that Nikita buys is 0.783.
(i) Find x.
(ii) Given that Nikita’s mother does not like her present, find the probability that the present is a scarf.
Solution
(i) To find the probability \(x\), we use the total probability that Nikita’s mother likes the present:
\(0.3 \times 0.72 + 0.7 \times x = 0.783\)
Simplifying, we get:
\(0.216 + 0.7x = 0.783\)
\(0.7x = 0.783 - 0.216\)
\(0.7x = 0.567\)
\(x = \frac{0.567}{0.7} = 0.81\)
(ii) To find the probability that the present is a scarf given that Nikita’s mother does not like it, we use conditional probability: