The moron who buy expired milk

Last lecture on AI today. Tutor go thru some past year exam papers answer as revision. 1 particular question caught my attention.

There are 5 containers of milk on a shelf. 2 of them have passed their use-by date. Suppose someone took 1 container, after examining the dates. What is the probability that you take 2 containers that are ahead of their used-by date?

Looks simple right?
So if the person took 1 container, that would means there are 2 expired and 2 non-expired milk.

Therefore the answer is: 2/4 * 1/3

Right???

WRONG!!!

According to the tutor, the person could had taken a expired milk or non-expired milk. Because the question did not state if it is expired or not expired. Therefore, we need to take the probability of him taking either of them.

But didn’t the question state that he took the milk after examining the dates? The probability of someone taking expired milk is ZERO.

Tried arguing with the tutor, but apparently, the model answer is taking assumption that the person could had bought either expired or non-expired milk. And if the question really want to make the 1st container random, then why do they have to say “after examining the dates” in the question? Why not just say someone took 1 bottle? It just doesn’t make sense. Besides…..

Which moron would buy expired milk?

I’m glad I didn’t take the exam that year…. 3 marks would had gone down the drain because of a stupid way of phrasing question.

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For those who are interested, the answer is:
2/4 * 1/3 + 3/4 * 2/3

2 comments

  1. “What is the probability that you take 2 containers that are ahead of their used-by date?”

    “According to the tutor, the person could had taken a expired milk or non-expired milk. Because the question did not state if it is expired or not expired.

    I thought the question asked for “expired” milk because the statement “are ahead of their used-by date” implies “expired milk“.

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