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# If the symbol @ represents either addition or multiplication

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10 Nov 2010, 03:56
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If the symbol @ represents either addition or multiplication, which operation does it represent?

(1) a@b=b@a for all numbers a and b
(2) a@(b–c)=(a@b)–(a@c) for all numbers a, b, and c
[Reveal] Spoiler: OA

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10 Nov 2010, 04:23
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zerotoinfinite2006 wrote:

[Reveal] Spoiler:
OA is "B", I am confused.

How come
a + ( b -c ) = (a + b) - (a + c) ?
and why not

a + b = b + a

If the symbol @ represents either addition or multiplication, which operation does it represent?

(1) a@b=b@a for all numbers a and b --> @ can be addition (a+b=b+a) as well as multiplication (a*b=b*a). Not sufficient.

(2) a@(b–c)=(a@b)–(a@c) for all numbers a, b, and c --> if @ represents addition we will have $$a@(b-c)=a+b-c$$ which is not equal to $$(a@b)-(a@c)=(a+b)-(a+c)=b-c$$, so @ must be multiplication. Sufficient. (Just to check: $$a@(b-c)=a*(b-c)=ab-ac$$ which is equal to $$(a@b)-(a@c)=ab-ac$$)

Similar problems:
symbol-problem-101741.html?hilit=symbol#p788840
ds-question-100955.html?hilit=symbol
just-800-level-question-99064.html?hilit=symbol

Hope it helps.
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21 Jun 2013, 02:50
Bumping for review and further discussion*. Get a kudos point for an alternative solution!

*New project from GMAT Club!!! Check HERE

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22 Jun 2013, 03:28
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zerotoinfinite2006 wrote:
If the symbol @ represents either addition or multiplication, which operation does it represent?

(1) a@b=b@a for all numbers a and b
(2) a@(b–c)=(a@b)–(a@c) for all numbers a, b, and c

Question: what is the operation @ ?
@ = + or *

As soon as you look at the questions, you will be reminded of the number properties Commutative and Distributive.

statement 1:
(1) a@b=b@a
Commutative property =>valid for both, + as well as *, =>cant say, Not Suff
(2) a@(b–c)=(a@b)–(a@c)
Distributive property=> Always valid for * -> Sufficient

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23 Mar 2015, 01:00
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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21 Nov 2015, 02:33
in statement B.. Why dont we consider the case of 0 where both +, x will work there by yielding E as answer? Is it because the Question says a,b,c are Numbers and hence we dont consider the case of 0? Which implies to consider 0 only when the it is mentioned to consider all integers/ or just integers. Therefore, if question mentions numbers it will be false to assume integers?

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22 Nov 2015, 02:29
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.

If the symbol @ represents either addition or multiplication, which operation does it represent?

(1) a@b=b@a for all numbers a and b
(2) a@(b–c)=(a@b)–(a@c) for all numbers a, b, and c

There is one variable (@) and 2 equations are given by the 2 conditions, so there is high chance (D) will be our answer.
For condition 1, @=*,+ , so this is insufficient.
For condition 2, @=*. This is sufficient, so the answer becomes (B).

For cases where we need 1 more equation, such as original conditions with “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. Since D is most likely to be the answer using 1) and 2) separately according to DS definition. Obviously there may be cases where the answer is A, B, C or E.
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23 Dec 2016, 03:17
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: If the symbol @ represents either addition or multiplication   [#permalink] 23 Dec 2016, 03:17
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