A fair 6-sided die

[didieravoaka]

I found A. Why C is the correct answer?

Thanks.

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Hi didieravoaka,

You can approach the math in this question in a couple of different ways, so you have to think about what would be the easiest way for YOU to organize your work.

The questions tells us to roll a 6-sided dice 3 times. We're asked for the probability of rolling EXACTLY one '3' on those three rolls.

Here's a way to break the calculation down into 3 smaller calculations:

(first roll is 3)(second roll is NOT 3)(third roll is NOT 3) = (1/6)(5/6)(5/6) = 25/216

(first roll is NOT 3)(second roll is 3)(third roll is NOT 3) = (5/6)(1/6)(5/6) = 25/216

(first roll is NOT 3)(second roll is NOT 3)(third roll is 3) = (5/6)(5/6)(1/6) = 25/216

Total = 3(25/216) = 75/216 = 25/72

Final Answer: C

GMAT assassins aren't born, they're made,
Rich

[ceilidh.erickson]

If you picked 25/216, that indicates that you weren't thinking about the order. You probably tested for (first roll is a 3)(second roll NOT a 3)(3rd roll NOT a 3).

On probability questions, you should ask yourself "what is this the probability of?" before answering the question. If you had realized that you had found (first roll is a 3)(2nd roll NOT a 3)(3rd roll NOT a 3), you could have realized that there were 2 other orders that would work: a 3 on the 2nd roll only, or a 3 on the 3rd roll only. You could then multiply 25/216 by 3, to get 75/216 --> 25/72.