A lottery is played by selecting X distinct single digit : GMAT Data Sufficiency (DS)
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# A lottery is played by selecting X distinct single digit

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A lottery is played by selecting X distinct single digit [#permalink]

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05 Feb 2013, 07:40
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A lottery is played by selecting X distinct single digit numbers from 0 to 9 at once such that order does not matter. What is the probability that a player will win playing the lottery?

1) Players must match at least two numbers with machine to win.

(2) X = 4
[Reveal] Spoiler: OA
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Re: A lottery is played by selecting X distinct single digit [#permalink]

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05 Feb 2013, 10:12
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danzig wrote:
A lottery is played by selecting X distinct single digit numbers from 0 to 9 at once such that order does not matter. What is the probability that a player will win playing the lottery?
(1) Players must match at least two numbers to win.
(2) X = 4

I'm happy to help.

This is a somewhat offbeat question, but then again, that's just what the GMAT will throw at you.

So, from the prompt, we know we are picking X different single digit numbers: X must be greater than two and less than 9 (or 10, if we are counting zero as a "single digit number" ----- let's ignore that complication). Order doesn't matter. We know nothing about what constitutes winning.

Statement #1: Players must match at least two numbers to win.
Now, at least we know what constitutes winning. The trouble is --- we don't know the how many digits are picked. If X = 9 ---- the lottery picks all the digits from 1-9, then I also pick all the digits from 1-9 --- then I have 100% chance of matching at least two digits and winning. That wouldn't be much of a lottery. If X = 3 --- the lottery picks three, and then I pick three --- well, that's harder. Clearly the probability of winning depends on the value of X, and we don't know that in Statement #1. This statement, alone and by itself, is insufficient.

Statement #2: X = 4
Now, we know how many digits are picked ---- lottery picks 4, then I pick 4 --- but now I have no idea what constitutes "winning". (This is an example of a DS question in which it's crucially important to forget all about Statement #1 when we are analyzing Statement #2 on its own.) In Statement #2, we know how many digits are picked, but we have absolutely no idea what constitutes winning. This statement, alone and by itself, is insufficient.

Combined statements:
Now, we know --- the lottery picks 4 digits, then I pick 4 digits, and if at least two of my digits match two of the lottery's cards, I win. This is now a well defined math problem, and if we wanted, we could calculate the numerical value of the probability. Of course, since this is DS, it would be a big mistake to waste time with that calculation. We have enough information now. Combined, the statements are sufficient.

Does all this make sense?

Mike
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Re: A lottery is played by selecting X distinct single digit [#permalink]

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05 Feb 2013, 11:49
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Hi,

I think that Mike's explanation is great. I'm just wondering where this question came from? The wording seems strange to the point that the question may not be solvable. Here is the potentially problematic wording: distinct single digit numbers from 0 to 9

What does that mean? Does that mean single digit INTEGERS between 0 and 9 or just single digit numbers. My assumption is that single digit numbers between 0 and 9 are not just "the digits" but could include the single digit decimals .1,.2,.3.... Anyone?

I guess that if you assume that the question includes the extra decimals then Mike's solution still works although I'm curious about whether this is an official question.

HG.
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Re: A lottery is played by selecting X distinct single digit [#permalink]

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07 Feb 2013, 08:22
HerrGrau wrote:
Hi,

I think that Mike's explanation is great. I'm just wondering where this question came from? The wording seems strange to the point that the question may not be solvable. Here is the potentially problematic wording: distinct single digit numbers from 0 to 9

What does that mean? Does that mean single digit INTEGERS between 0 and 9 or just single digit numbers. My assumption is that single digit numbers between 0 and 9 are not just "the digits" but could include the single digit decimals .1,.2,.3.... Anyone?

I guess that if you assume that the question includes the extra decimals then Mike's solution still works although I'm curious about whether this is an official question.

HG.

It's tagged as a grockit problem, so I would guess that this is not an official question. It is also tagged as a 600-700 lvl question, which seems high to me for this question...I don't have any experience with Grockit, but MGMAT questions that are in the 600-700 range tend to be more difficult for me to solve.
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Re: A lottery is played by selecting X distinct single digit [#permalink]

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07 Feb 2013, 10:09
HerrGrau wrote:
Hi,
The wording seems strange to the point that the question may not be solvable. Here is the potentially problematic wording: distinct single digit numbers from 0 to 9 What does that mean? Does that mean single digit INTEGERS between 0 and 9 or just single digit numbers. My assumption is that single digit numbers between 0 and 9 are not just "the digits" but could include the single digit decimals .1,.2,.3.... Anyone?

Dear HerrGrau
Thank you for your kind words.

In my experience, I have never heard "single digit numbers" apply to anything other than non-negative integers. I have never seen any book or hear anyone ever refer to, say, -5 or 0.008 or 3 x 10^8 as a "single digit number. Yes, technically, each of these is written with a single non-zero digit, but I have never heard the term "single digit number" used for them. (We could say that each of these has "one significant figure", but that carries us far afield into measurement theory, well beyond GMAT territory.)

In my mind, the only ambiguity in this question is whether zero is included --- I guess I was assuming, whatever their understanding of the term, this understanding would be fixed and wouldn't change as we moved through the statements of the DS ---- therefore, with both statements, and with whatever convention they are following, the question can be definitely answered.

I agree with you ---- this question is not written with the tight precision so characteristic of official GMAT questions. The very fact that there's any ambiguity at all makes this a woefully substandard question.

Mike
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Re: A lottery is played by selecting X distinct single digit [#permalink]

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07 Feb 2013, 14:47
Hi Mike,

In general I agree with everything that you're saying. Just to clarify, I think that .008 is a three digit number with 2 unique digits and so would be excluded. The numbers in question are .1, .2, .3, .4, .5, .6, .7, .8, and .9. They are numbers that have only one digit and hence are "single digit numbers". I have never seen this distinction be an actual issue on a GMAT question but am curious as to what the general consensus is.

HG.
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26 Jul 2014, 01:46
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Re: A lottery is played by selecting X distinct single digit [#permalink]

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16 Oct 2014, 11:04
just for the sake of understanding, how would we solve this question?
1- probability of matching none - prob of matching 1?
none= (5/9)*4/9*3/9*2/9

probability of matching one: 1*5/9*4/9*3/9

is this the way?

Thank you!
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02 Dec 2015, 07:31
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Re: A lottery is played by selecting X distinct single digit [#permalink]

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05 Dec 2015, 11:47
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Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.

A lottery is played by selecting X distinct single digit numbers from 0 to 9 at once such that order does not matter. What is the probability that a player will win playing the lottery?

1) Players must match at least two numbers to win.

(2) X = 4

There are 2 variables (x and how many to be win) and 2 equations are given, so there is high chance (C) will be the answer.
Looking at the conditions together,
the probability to win is 1-(10C4/10^4), which is unique and makes the condition sufficient,

For cases where we need 2 more equations, such as original conditions with “2 variables”, or “3 variables and 1 equation”, or “4 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 70% chance that C is the answer, while E has 25% chance. These two are the majority. In case of common mistake type 3,4, the answer may be from A, B or D but there is only 5% chance. Since C is most likely to be the answer using 1) and 2) separately according to DS definition (It saves us time). Obviously there may be cases where the answer is A, B, D or E.
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Re: A lottery is played by selecting X distinct single digit   [#permalink] 05 Dec 2015, 11:47
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