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Is ab = 9c^2–4?

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Is ab = 9c^2–4?  [#permalink]

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New post 12 Feb 2015, 07:07
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A
B
C
D
E

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  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.
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Re: Is ab = 9c^2–4?  [#permalink]

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New post 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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New post 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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New post 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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New post 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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New post 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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New post 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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New post 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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New post 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

no info about

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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New post 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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