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In this question, we're given at a store, 20%, 30 % and 50 % of phones are manufactured by factory a, b and c respectively.
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2%, 1 % and 0 .05 % of defective phones are from a, b and c respectively.
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And a customer asks for refund for defective phone.
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I'm going to let a denote the event the phone is from factory a.
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B denote the event, the phones from factory b.
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C denote the event, the phones from factory c.
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And d did not the event, the phone is defective.
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Now let's draw the probability tree.
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Probability the phone is from factory a is 20 % or 0 .2 in decimal.
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Probability the phones from factory b is 0 .3.
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That is 30%.
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And probability the phone is from factory c is 50 % or 0 .5.
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Now they are going to each branch off to event d, which is the phone is defective, or d prime or d complement, the phone is not defective.
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Now over here, this portion here is the phone is defective given that it comes from factory a.
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So that will be 2 % or 0 .02.
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Over here, this is probability the phone is defective given comes from b, and that is 1 % or 0 .01.
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And over here this probability the phone is defective given it comes from c is 0 .05 that will be 0 .005 in decimal.
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In part a we want to find probability the phone is defective.
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So we want to find probability of d.
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That will be taking three mutually exclusive case.
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The first case is a and d.
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Now n is times set notation is intercept or is plus set notation is union...