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Math Expert V
Joined: 02 Sep 2009
Posts: 59561

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Difficulty:   15% (low)

Question Stats: 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.
Manager  Joined: 03 Oct 2014
Posts: 126
Location: India
Concentration: Operations, Technology
GMAT 1: 720 Q48 V40 WE: Engineering (Aerospace and Defense)
Re: Is ab = 9c^2–4?  [#permalink]

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C - Using A & B....A= 3c+2, B= 3c-2
Manager  Joined: 14 Jul 2014
Posts: 88
Re: Is ab = 9c^2–4?  [#permalink]

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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
Intern  Joined: 22 Aug 2014
Posts: 38
Re: Is ab = 9c^2–4?  [#permalink]

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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
Math Expert V
Joined: 02 Sep 2009
Posts: 59561
Re: Is ab = 9c^2–4?  [#permalink]

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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).
EMPOWERgmat Instructor V
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GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: Is ab = 9c^2–4?  [#permalink]

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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

GMAT assassins aren't born, they're made,
Rich
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Posts: 1
Re: Is ab = 9c^2–4?  [#permalink]

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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.
Math Expert V
Joined: 02 Sep 2009
Posts: 59561
Re: Is ab = 9c^2–4?  [#permalink]

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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.
Director  S
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Location: United States
Schools: Yale '18
GMAT 1: 650 Q43 V37 GRE 1: Q157 V158 GPA: 2.66
Re: Is ab = 9c^2–4?  [#permalink]

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