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nislam
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nislam
At an automotive plant, 2 % of cars produced have faulty braking systems. Each automobile undergoes a braking system check. When a car has a faulty braking system, this check correctly identifies the fault 90 % of the time. When a car does not have a faulty braking system, the check incorrectly identifies the system as faulty 5 % of the time. If the check reports that the braking system of a certain car is faulty, what is the probability that the braking system of this car is actually faulty?

A) \(\frac{9}{50}\)

B) \(\frac{18}{67}\)

C) \(\frac{41}{50}\)

D) \(\frac{49}{67}\)

E) \(\frac{499}{500}\)

nislam

Is there a piece of information that you would just LOVE to know that's missing from the question? What if we knew the total number of cars? Wouldn't knowing that make our lives much easier? Then let's make it up!! (I call these Hidden Plug In questions, and if you search for ThatDudeKnowsHiddenPlugIn, you'll find other examples.)

Let's say there are 1000 cars.
2% have faulty braking systems, so 20. The check correctly identifies 90%, so 18, of those. 18 times we're told there is a problem and there actually is a problem.
When a car does not have a faulty braking system (that's 980 cars), 5% of those say there is a problem. 5% of 980 is 49. 49 times we're told there is a problem and there actually is NOT a problem.
There is a total of 18+49=67 times when we are told there is a problem. Of those, 18 times there is actually a problem.
16/67
Answer choice B.


ThatDudeKnowsHiddenPlugIn
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