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If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)

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If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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03 Jul 2020, 00:59
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If ab - 2b = (4 - a)b, what is the value of b?

(1) |a^2 - 9| ≤ 0
(2) a < 0

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Re: If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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03 Jul 2020, 01:36
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1
Quote:
If ab - 2b = (4 - a)b, what is the value of b?

(1) |a^2 - 9| ≤ 0
(2) a < 0

ab - 2b = 4b -ab
2ab - 6b = 0
2b (a - 3) = 0

statement 1: |a^2 - 9| ≤ 0
a^2 - 9 = 0
a can be +3 or -3
when a = 3, b can be anything.
when a= -3, b has to be 0.
not sufficient

statement 2: a < 0
a $$\ne 0$$
b has to be 0.
ans: B
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Re: If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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03 Jul 2020, 02:05
1
ab - 2b = (4 - a)b
6b = 2ab
3b = ab
b(3-a) = 0

St. 1:
|a^2 - 9| <= 0
a^2 - 9 <= 0; when a^2 - 9 <= 0 i.e; a -> [-3,3]..........(i)
a^2 - 9 >=0; when a^2 - 9 >= 0 i.e; a -> (-infinity, - 3] U [3, inifnity)..........(ii)

From (i):
if a = 3, b = any value
if a = 2, b = 0
if a = 1, b = 0

St. 1 is insufficient

St. 2:
a < 0
=> a != 3
=> a - 3 != 0
=> b = 0
St. 2 is sufficient

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Re: If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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03 Jul 2020, 03:04
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Solution

Step 1: Analyse Question Stem

• We have, $$ab-2b = (4-a)b$$

• We need to find the value of b.

• Now, $$ab-2b = (4-a)b$$
$$⟹ b(a-2) – b(4-a) = 0$$
$$⟹ b( a- 2 – 4 + a)= 0$$
$$⟹ b(2a – 6)= 0$$
$$⟹ 2b(a-3)= 0 …….Eq.(i)$$
 So, if $$a ≠ 3$$, in that case b must be 0.
 And, if $$a = 3$$, in that case b can take any real number value.
• So, if we can determine that $$a ≠ 3$$, we can find the unique value of b.
Now, let’s analyse the statements.

Step 2: Analyse Statements Independently (And eliminate options) – AD/BCE

Statement 1: $$|a^2 – 9| ≤ 0$$
• We know that absolute values cannot be less than 0
o This, means, $$|a^2 – 9| = 0$$ $$⟹ a = -3,$$ or $$3$$
 If a = 3, in that case b = 0, unique solution
 However, if a = -3, in that cases b can take any real number value.
Hence, statement 1 is NOT sufficient and we can eliminate answer Options A and D.

Statement 2: $$a< 0$$
• From this statement, we can be sure that $$a ≠ 3$$.
o Therefore, $$b = 0$$
Hence, statement 2 is sufficient.
Thus, the correct answer is Option B.
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Re: If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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03 Jul 2020, 03:24
from question:
ab-2b=4b-ab
2ab=6b

statement 1: tells us either a=<3, -3 or -3, 3<=a, nothing about B, hence Insuff
statement 2: tells us nothing about b, hence insuff
together: nothing about b.. hence IMO ans: E
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If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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Updated on: 06 Jul 2020, 04:54
If ab - 2b = (4 - a)b, what is the value of b?

(1) |a^2 - 9| ≤ 0
(2) a < 0

ab - 2b = 4b - ab
2ab = 6b
2ab - 6b = 0
b(a - 3) = 0

So, either b = 0 and a - 3 ≠ 0
OR
b ≠ 0 and a - 3 = 0
OR
b = 0 and a - 3 = 0

(1) |a^2 - 9| ≤ 0
Since |mod| ≥ 0
|a^2 - 9| = 0 only
a^2 - 9 = 0
a = 3 => b can take any value
OR -3 => b = 0
However, b can take any value.

INSUFFICIENT.

(2) a < 0
Here, since a < 3 so b can take only one value i.e.
b = 0, so that b(a - 3) = 0

SUFFICIENT.

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Originally posted by unraveled on 03 Jul 2020, 04:11.
Last edited by unraveled on 06 Jul 2020, 04:54, edited 1 time in total.
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Re: If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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03 Jul 2020, 05:03
1
Given: ab - 2b = (4 - a)b which upon simplification becomes b(3-a)=0. Thus, either b = 0 or a = 3 or both.

Statement 1: |a^2 - 9| ≤ 0, as absolute (modulus) values are either 0 or Positive Numbers. Therefore a^2 = 9.
We get a = 3 or a = -3. Thus we cannot be sure about the value of 'b'. Insufficient

Statement 2: a < 0, thus a≠3. Therefore, b = 0. Sufficient.

Choice B is the correct answer.
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Re: If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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03 Jul 2020, 10:17
If ab - 2b = (4 - a)b, what is the value of b?

(1) |a^2 - 9| ≤ 0
(2) a < 0

Given, b (a-2) = b (4 -a), so, when b =0, a can assume any value. If b is not 0, a will be 3.

1) a^2 - 9 =0, as the absolute value cannot be less than 0. so, a is either -3 or +3. Both of the values will satisfy the equation given b is 0 or not. not sufficient.

2) no information about b. not sufficient.

Together, a can assume only the value -3. so b is 0. Sufficient.

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Re: If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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03 Jul 2020, 12:58
(a - 2)b = (4 - a)b
Either a=3 or b=0

Q. what is the value of b?

(1) |a^2 - 9| ≤ 0
a=-3 --> b=0
a=3 --> b can be any
NOT SUFFICIENT

(2) a < 0
a=-3 --> b=0
Any other a --> b=0
SUFFICIENT

Ans (B)

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Re: If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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03 Jul 2020, 16:16
E - Neither sufficient

ab - 2b = (4 - a)b ==> ab - 2b = 4b - ab ==> 2ab = 6b. ==> a = 3

then 3b - 2b = (4-3)b ===> b=b

So whatever value we chose for B, will give us B and since neither statement 1 or 2 talk about option B we can conclude the answer is E
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Re: If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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03 Jul 2020, 22:20
If ab - 2b = (4 - a)b, what is the value of b?

(1) |a^2 - 9| ≤ 0
(2) a < 0

This is a question in which the question stem gives more information than we usually give it credit for!

ab - 2b = 4b - ab
=> 2ab - 6b = 0
=> (a-3)b = 0

So either a = 3 or b = 0

st1) From the inequality equation, it is clear that $$a\neq{3}$$, so b has to be equal to 0 (SUFFICIENT)

st2) clearly tells us that $$a\neq{3}$$ , so b = 0 (SUFFICIENT)

So, the answer should be D
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Re: If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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03 Jul 2020, 22:43
1
IMO B.

ab-2b = 4b-ab

2ab = 4b + 2b

2ab =6b

ab =3b

ab-3b = 0
b(a-3) = 0

either a=3 or b= 0 or both

Stmt 1
suggests that a^2 = 9
since it is absolute
a = 3

(a = -3 does not hold true after we simplify the question stem.)

3b = 3b

we get b = b
We dont get value so insuff.

Stmt 2 says a is negative.
Since this condition is false after we simplify the question stem.
so b has to be 0.
Suff.

B it is

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Re: If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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03 Jul 2020, 22:51
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From statement 1
a =3, so b can take up 0 or any other value
Not sufficient
From statement 2
a is less than 0 that means b can only be 0.
Sufficient

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If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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Updated on: 09 Jul 2020, 03:39
We have $$ab-2b=(4-a)b$$

$$(a-2)b=(4-a)b$$
$$ab-2b=4b-ab$$
$$2ab-6b=0$$
$$ab-3b=0$$
$$b(a-3)=0$$

From the above we can infer the following two cases -
(i) if $$a=3$$, $$b$$ can be any real number
(ii) if $$a\neq{3}$$, $$b$$ = 0

From Statement (1) we have,
$$|a^2 - 9|\leq{0}$$
We know that a modulus function always gives a positive number as it's output. Hence $$|a^2 - 9|$$ cannot be negative and can only be equal to 0 at the best.

$$a^2-9=0$$
$$a^2=9$$
$$a=3$$ or $$a=-3$$

Unique value of b cannot be arrived at by either value of a. Hence Options (A) and (D) may be eliminated.

From Statement (2) we have,
$$a<0$$

If $$a<0$$ then $$a\neq{3}$$. Then b should be equal to 0 as we have found in (ii) above. Since we have a unique value of b in this case, (B) is the answer.

Originally posted by Pran1990 on 04 Jul 2020, 05:05.
Last edited by Pran1990 on 09 Jul 2020, 03:39, edited 1 time in total.
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Re: If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)  [#permalink]

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05 Jul 2020, 10:02
And C

With statement A
You get two values of a= 3, -3
This is not sufficient to find b

With statement 2
A<0
We are not clear what A is and what can be possible value for b. So this is insufficient

1+2
it is clear that A= -3, as A<0, with this find B from given equation
-3B -2B = -5B = 7B
So B=0

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Re: If ab - 2b = (4 - a)b, what is the value of b? (1) |a^2 - 9| ≤ 0 (2)   [#permalink] 05 Jul 2020, 10:02