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# Is ab = 9c^2–4?

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Math Expert
Joined: 02 Sep 2009
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12 Feb 2015, 07:07
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78% (01:34) correct 22% (01:49) wrong based on 174 sessions

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Is ab = 9c^2–4?

(1) a divided by 3 is equal to c with a remainder of 2.
(2) b divided by 3 is equal to c minus 2/3.

Kudos for a correct solution.

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Re: Is ab = 9c^2–4?  [#permalink]

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13 Feb 2015, 00:22
1
Ans C

Stmnt 1 -Nothing about b - Not Suff
Stmnt 2 - Nothing about a - Not Suff

Combining, a = 3c + 6 and b = 3c - 2
Possible to compute and answer the Question - Hence Suff !

Ans - C
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Re: Is ab = 9c^2–4?  [#permalink]

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13 Feb 2015, 01:29
1
Bunuel wrote:
Is ab = 9c^2–4?

(1) a divided by 3 is equal to c with a remainder of 2.
(2) b divided by 3 is equal to c minus 2/3.

Kudos for a correct solution.

1. a = 3c+6
2. b = 3c-2

therefore ab = (3c+6)*(3c-2) = 9C^2+12C-12 <> 9C^2 - 4

Option C
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Re: Is ab = 9c^2–4?  [#permalink]

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18 Jul 2016, 23:56
1
ajxxatic wrote:
Bunuel wrote:
Bunuel wrote:
Is ab = 9c^2–4?

(1) a divided by 3 is equal to c with a remainder of 2.
(2) b divided by 3 is equal to c minus 2/3.

Kudos for a correct solution.

VERITAS PREP OFFICIAL SOLUTION

Solution: C

Statement (1) says nothing about b; INSUFFICIENT. Statement (2) says nothing about a; INSUFFICIENT. When combining the statements, be sure to simplify them first. If a/3 = c with a remainder of 2, then a = 3c + 2. If b/3 = c – 2/3, then b = 3c-2. Thus ab = (3c+2)(3c-2), or 9c^2–4; (C).

Hi,
I understand that a = 3c+2 but how is B = 3c-2? B leaves a remainder of -2/3. Please double check.

(2) b divided by 3 is equal to c minus 2/3.

b/3 = c - 2/3;

Multiply by 3: b = 3c - 2.

Hope it's clear.
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Re: Is ab = 9c^2–4?  [#permalink]

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12 Feb 2015, 07:58
C - Using A & B....A= 3c+2, B= 3c-2
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Re: Is ab = 9c^2–4?  [#permalink]

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16 Feb 2015, 06:18
Bunuel wrote:
Is ab = 9c^2–4?

(1) a divided by 3 is equal to c with a remainder of 2.
(2) b divided by 3 is equal to c minus 2/3.

Kudos for a correct solution.

VERITAS PREP OFFICIAL SOLUTION

Solution: C

Statement (1) says nothing about b; INSUFFICIENT. Statement (2) says nothing about a; INSUFFICIENT. When combining the statements, be sure to simplify them first. If a/3 = c with a remainder of 2, then a = 3c + 2. If b/3 = c – 2/3, then b = 3c-2. Thus ab = (3c+2)(3c-2), or 9c^2–4; (C).
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Re: Is ab = 9c^2–4?  [#permalink]

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19 Feb 2015, 20:16
Hi buddyisraelgmat and aj13783,

While you both selected the correct answer, you both made the same "math mistake" in your work. While that error didn't hurt you on this question, it might impact other questions in the future, so it's worth 'fixing.'

In Fact 1, we're told that when A is divided by 3 equals C with a remainder of 2. This translates into:

A/3 = C remainder 2

If you want to isolate the A, then you multiply both sides by 3, BUT you do NOT multiply the remainder (since the remainder represents the "leftover pieces" and that part does NOT get tripled).

So

A = 3C + 2 (not A = 3C + 6)

Here is an example of that, using real numbers.....

Let's say A = 14

14/3 = 4 remainder 2

14 = 3(4) + 2

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Re: Is ab = 9c^2–4?  [#permalink]

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18 Jul 2016, 12:37
Bunuel wrote:
Bunuel wrote:
Is ab = 9c^2–4?

(1) a divided by 3 is equal to c with a remainder of 2.
(2) b divided by 3 is equal to c minus 2/3.

Kudos for a correct solution.

VERITAS PREP OFFICIAL SOLUTION

Solution: C

Statement (1) says nothing about b; INSUFFICIENT. Statement (2) says nothing about a; INSUFFICIENT. When combining the statements, be sure to simplify them first. If a/3 = c with a remainder of 2, then a = 3c + 2. If b/3 = c – 2/3, then b = 3c-2. Thus ab = (3c+2)(3c-2), or 9c^2–4; (C).

Hi,
I understand that a = 3c+2 but how is B = 3c-2? B leaves a remainder of -2/3. Please double check.
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Re: Is ab = 9c^2–4?  [#permalink]

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17 Jul 2017, 18:06
Bunuel wrote:
Is ab = 9c^2–4?

(1) a divided by 3 is equal to c with a remainder of 2.
(2) b divided by 3 is equal to c minus 2/3.

Kudos for a correct solution.

Before looking at the statements we can easily see that this question is just a variation of (a+b)(a-b) - a common theme on the GMAT- (3b-2)(3b +2)

Statement 1

If a/3 = c remainder 2 then = a= 3c +2

but no info about "b" insuff

Statement 2

Same thing

b/3= c-2/3 - use algebra
b= 3c- 2

a

St 1 & 2

(3c-2)(3c +2) suff

c
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Re: Is ab = 9c^2–4?  [#permalink]

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19 Mar 2019, 09:37
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Re: Is ab = 9c^2–4?   [#permalink] 19 Mar 2019, 09:37
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