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by MartyMurray » Sat Apr 16, 2016 5:48 am
eitijan wrote:I am confused between option B & C
Some of the wording of the question seems to be missing in the screen shot, but from the answer choices one can gather that the probability of any button failing at 4 months is 16/25.

So if Lisa wants to dial a particular 7 digit number, the probability that all seven buttons used for that number will have failed is (16/25)�.

However, even if one of the buttons has failed, she won't be able to dial the number. So choice B, 1 - (16/25)�, is too high as 1 - (16/25)� is the probability that not all of the buttons will have failed.

What you need is the probability that NONE of the buttons have failed.

If the probability of one button failing is 16/25, then the probability of a button not having failed is 9/25. So the probability that seven buttons have not failed is (9/25)�.

The correct answer is C.
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