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Board of Directors D
Joined: 01 Sep 2010
Posts: 3408
If a/b=2/3 which of the following is not true ?  [#permalink]

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Difficulty:   5% (low)

Question Stats: 80% (01:07) correct 20% (01:18) wrong based on 223 sessions

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If $$\frac{a}{b}=\frac{2}{3}$$ which of the following is not true ?

A) $$\frac{a+b}{b}=\frac{5}{3}$$

B) $$\frac{b}{b-a}=3$$

C) $$\frac{a-b}{b}=\frac{1}{3}$$

D) $$\frac{2a}{3b}=\frac{4}{9}$$

E) $$\frac{a+3b}{a}=\frac{11}{2}$$

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Re: If a/b=2/3 which of the following is not true ?  [#permalink]

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carcass wrote:
If $$\frac{a}{b}=\frac{2}{3}$$ which of the following is not true ?

A) $$\frac{a+b}{b}=\frac{5}{3}$$

B) $$\frac{b}{b-a}=3$$

C) $$\frac{a-b}{b}=\frac{1}{3}$$

D) $$\frac{2a}{3b}=\frac{4}{9}$$

E) $$\frac{a+3b}{a}=\frac{11}{2}$$

One of the possible values of a and b is a = 2 and b = 3. In this case all options are true but C:

$$(\frac{2-3}{3}=-\frac{1}{3})\neq \frac{1}{3}$$.

Similar question with several different approaches: https://gmatclub.com/forum/if-5x-4y-whi ... 34924.html
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Re: If a/b=2/3 which of the following is not true ?  [#permalink]

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The fastest approach is just the aforementioned.

a= and b=3 and substitution. ASAP you find the value different, that is the answer.

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Re: If a/b=2/3 which of the following is not true ?  [#permalink]

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carcass wrote:
If $$\frac{a}{b}=\frac{2}{3}$$ which of the following is not true ?

A) $$\frac{a+b}{b}=\frac{5}{3}$$

B) $$\frac{b}{b-a}=3$$

C) $$\frac{a-b}{b}=\frac{1}{3}$$

D) $$\frac{2a}{3b}=\frac{4}{9}$$

E) $$\frac{a+3b}{a}=\frac{11}{2}$$

We are given that a/b = 2/3 or 3a = 2b.

Let’s simplify each answer choice to determine which one cannot be manipulated to yield 3a = 2b.

A) (a + b)/b = 5/3

3(a + b) = 5b

3a + 3b = 5b

3a = 2b

Since we have 3a = 2b, answer choice A is not correct.

B) b/(b - a) = 3

b = 3(b - a)

b = 3b - 3a

3a = 2b

Since we have 3a = 2b, answer choice B is not correct.

C) (a - b)/b = 1/3

3(a - b) = b

3a - 3b = b

3a = 4b

Since we have 3a = 4b, answer choice C is correct.

Alternative solution:

Let’s test each answer choice to determine which one is not true (keep in mind that a/b = 2/3).

A) (a + b)/b = 5/3

(a + b)/b = a/b + b/b = 2/3 + 1 = 5/3 → This is true.

B) b/(b - a) = 3

Notice that b/(b - a) = 3 means (b - a)/b = 1/3 when we reciprocate both sides of the equation.

(b - a)/b = b/b - a/b = 1 - 2/3 = ⅓ → This is true.

C) (a - b)/b = 1/3

(a - b)/b = a/b - b/b = 2/3 - 1 = -1/3 → So (a - b)/b = 1/3 is not true. This is the correct answer choice.

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Director  G
Joined: 09 Mar 2018
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Re: If a/b=2/3 which of the following is not true ?  [#permalink]

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carcass wrote:
If $$\frac{a}{b}=\frac{2}{3}$$ which of the following is not true ?

A) $$\frac{a+b}{b}=\frac{5}{3}$$

B) $$\frac{b}{b-a}=3$$

C) $$\frac{a-b}{b}=\frac{1}{3}$$

D) $$\frac{2a}{3b}=\frac{4}{9}$$

E) $$\frac{a+3b}{a}=\frac{11}{2}$$

use values, a/b = 2/3

a = 2b / 3, a =2 b = 3

Only C doesn't satisfy when those values are used.
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Joined: 03 Jun 2019
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If a/b=2/3 which of the following is not true ?  [#permalink]

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carcass wrote:
If $$\frac{a}{b}=\frac{2}{3}$$ which of the following is not true ?

A) $$\frac{a+b}{b}=\frac{5}{3}$$

B) $$\frac{b}{b-a}=3$$

C) $$\frac{a-b}{b}=\frac{1}{3}$$

D) $$\frac{2a}{3b}=\frac{4}{9}$$

E) $$\frac{a+3b}{a}=\frac{11}{2}$$

Given: $$\frac{a}{b}=\frac{2}{3}$$

Asked: which of the following is not true ?

A) $$\frac{a+b}{b}=\frac{5}{3}$$
$$\frac{a+b}{b}= \frac{2+3}{3}= \frac{5}{3}$$
TRUE

B) $$\frac{b}{b-a}=3$$
$$\frac{b}{b-a}=3(3-2) = 3$$
TRUE

C) $$\frac{a-b}{b}=\frac{1}{3}$$
$$\frac{a-b}{b}=\frac{2-3}{3} = -\frac{1}{3} \neq \frac{1}{3}$$
NOT TRUE

D) $$\frac{2a}{3b}=\frac{4}{9}$$
$$\frac{2a}{3b}=\frac{2*2}{3*3}= \frac{4}{9}$$
TRUE

E) $$\frac{a+3b}{a}=\frac{11}{2}$$
$$\frac{a+3b}{a}= = \frac{2+3*3}{2} = \frac{11}{2}$$
TRUE

IMO C
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