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# 2 gmatprep PS

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Manager
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07 Mar 2007, 11:32
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0% (00:00) correct 0% (00:00) wrong based on 0 sessions

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SVP
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07 Mar 2007, 13:09
Question 2)

(D) for me

o [9] = 3*9 = 27
o [6] = 1/2*6 = 3

[9] * [6] = 27 * 3 = 81

Then, looking at every anwser response... We could eliminate [81] and all evens because they are divided by 2 and below 81... So [54], [36] and [18] are out...

Finally, to make a last check, we have :
[27] = 27 * 3 = 81
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07 Mar 2007, 13:59
Q1. E (i do not know how the program picked up the other anwer)
Q2. D
Manager
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07 Mar 2007, 14:13
1)

Given a@b = a+b-ab

[I] a@b = b@a
[II] a@0 = a
[III] (a@b)@c = a@(b@c)

[I]
Left hand side is:
a+b-ab
Right hand side is:
b+a-ba

Both sides are equal.

[II]
Left hand side is:
a+0-a*0 = a
Right hand side is:
a

Both sides are equal.

[III]
Left hand side is:
(a+b-ab)@c=a+b-ab+c-(a+b-ab)*c=a+b-ab+c-ac-bc+abc=a+b+c-ab-bc-ca+abc
Right hand side is:
a@(b+c-bc)=a+b+c-bc-a*(b+c-bc)=a+b+c-bc-ab-ac+abc=a+b+c-ab-bc-ca+abc

Both sides are equal.

Manager
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07 Mar 2007, 14:15

[9] = 3*9 = 27
[6] = 1/2 * 6 = 3

[9]*[6]=27*3=81
Senior Manager
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07 Mar 2007, 18:57
I go with

Q1. E
Q2. D

rdg,
for question 2 answers also need to be applied with same rule for ODD as mentioned in Question stem.
So if you pick answer A) which is 81 inside a square then value for it is actually 81*3 which is not what you calculated, right?
So answer is D which is equivalent of 27 * 3 = 81
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07 Mar 2007, 21:47
kyatin wrote:
I go with

Q1. E
Q2. D

rdg,
for question 2 answers also need to be applied with same rule for ODD as mentioned in Question stem.
So if you pick answer A) which is 81 inside a square then value for it is actually 81*3 which is not what you calculated, right?
So answer is D which is equivalent of 27 * 3 = 81

Thanks kyatin.
I understand what you meant. It will be (D).
Senior Manager
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08 Mar 2007, 22:00

(1) D - I and III are TRUE

(2) A - 81

-------------------------
Q1 Here I show how I solved it:

a@b = a + b - ab

a@b = a - ab + b = a (-ab+) b

which means @ = (-ab+) .....then I checked it with all three I, II and III and found I and III are TRUE.

----------------------------
Q2: Answer is 81 which is (A) then why is everybody writing it (D)?
Manager
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09 Mar 2007, 05:54
Lets do backward calc.

if, [m]=3m, when odd, result of [81]=243.
Now if [9]x[6]=[81] true, [9]x[6]=243

but

if [9] x [6] = [27], the result [27]=27x3=81

If the answer choice (A) is 81 instead of [81], then (A) must be true.
Manager
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09 Mar 2007, 06:02
Summer3 wrote:

(1) D - I and III are TRUE

(2) A - 81

-------------------------
Q1 Here I show how I solved it:

a@b = a + b - ab

a@b = a - ab + b = a (-ab+) b

which means @ = (-ab+) .....then I checked it with all three I, II and III and found I and III are TRUE.

----------------------------
Q2: Answer is 81 which is (A) then why is everybody writing it (D)?

(II) a@0=a

if @=(-ab+), when a@b=a+b-ab, then

a@0 = a (-a*0+)0 =a(-0+0) = a

So (II) must be true as well.
Senior Manager
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09 Mar 2007, 07:00
Thanks rdg!

The second question:

[9] * [6] = (3*9) * (6/2) = 81

But 81 and [81] are different so answer is NOT (A).

81 is an odd number so 3m =[m] will be applicable here.

81 = 3m = 3*27 = [27]

Last edited by Summer3 on 09 Mar 2007, 07:09, edited 2 times in total.
Senior Manager
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09 Mar 2007, 07:07
I checked the first question too. You are right, rdg! All I, II and III are TRUE.

Thanks!
09 Mar 2007, 07:07
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