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Probability of getting # 1

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Probability of getting # 1

by Vemuri » Sun Mar 22, 2009 8:05 am
When a die that has one of six consecutive integers on each of its sides is rolled twice, what is the probability of getting the number 1 on both rolls?

1. The probability of NOT getting an eight is 1.
2. The probability of NOT getting a seven is 25/36.
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Source: — Data Sufficiency |
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by DanaJ » Sun Mar 22, 2009 8:23 am
1. tells us that 8 is not on the sides of the dice, since we're 100% sure that 8 can't be there. However, this doesn't help establish whether 1 is on the sides of the dice. Consider these possibilities for the sides of the dice:
a. 1, 2, 3, 4, 5, 6
b. 9, 10, 11, 12, 13, 14

In both cases, 8 isn't there. This is why 1 is insufficient.

2. tells us that 7 is indeed on one of the sides of the dice. This, IMHO, is sufficient to answer your question, since anyway you take it, 1 is "too far away" from 7. Consider the best case: 7 and the numbers immediately smaller than 7:
You get 7, 6, 5, 4, 3, 2 and then you need to stop, since you already have 6 numbers: you are just one number short from 1.

This is why the answer is IMO B
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