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# If 12a + 3b = 1 and 7b – 2a = 9, what is the average (arithmetic mean)

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Math Expert
Joined: 02 Sep 2009
Posts: 56300
If 12a + 3b = 1 and 7b – 2a = 9, what is the average (arithmetic mean)  [#permalink]

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17 Aug 2018, 06:01
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Difficulty:

25% (medium)

Question Stats:

79% (02:07) correct 21% (02:35) wrong based on 36 sessions

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If 12a + 3b = 1 and 7b – 2a = 9, what is the average (arithmetic mean) of a and b ?

A) 0.1
B) 0.5
C) 1
D) 2.5
E) 5

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Re: If 12a + 3b = 1 and 7b – 2a = 9, what is the average (arithmetic mean)  [#permalink]

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17 Aug 2018, 06:09
Multiplying equation 2 by 6 gives 42b-12a=54.
Now adding this to eqn 1 gives 45b=55. Hence b = $$\frac{11}{9}$$.
replacing this in eqn 1 gives a = $$\frac{-2}{9}$$.
The average of a+b gives 0.5.
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If 12a + 3b = 1 and 7b – 2a = 9, what is the average (arithmetic mean)  [#permalink]

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17 Aug 2018, 06:10
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1
Bunuel wrote:
If 12a + 3b = 1 and 7b – 2a = 9, what is the average (arithmetic mean) of a and b ?

A) 0.1
B) 0.5
C) 1
D) 2.5
E) 5

12a+3b+7b-2a=10
Or, 10a+10b=10
Or, 10(a+b)=10
Or, a+b=1
So, average of a and b is 0.5.

Ans. (B)
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If 12a + 3b = 1 and 7b – 2a = 9, what is the average (arithmetic mean)  [#permalink]

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17 Aug 2018, 06:57
Bunuel wrote:
If 12a + 3b = 1 and 7b – 2a = 9, what is the average (arithmetic mean) of a and b ?

A) 0.1
B) 0.5
C) 1
D) 2.5
E) 5

In order to find the arithmetic mean, we need the value of a+b

P.S In the GMAT, most of the times, this kind of question can be done by a very straightforward
approach. You either have to add or subtract the two statements. Sometimes, you might have
to add twice the second statement to the first statement. Try these combinations before you go
ahead and find individual values for a & b.

12a + 3b + 7b – 2a = 9 + 1 -> 10(a + b) = 10 -> a + b = 1

Therefore, the average of a and b is $$\frac{a+b}{2} = \frac{1}{2}$$ = 0.5(Option B)
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Re: If 12a + 3b = 1 and 7b – 2a = 9, what is the average (arithmetic mean)  [#permalink]

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18 Aug 2018, 09:55
10a + 10b = 10
a + b = 1
so arithmetic mean is 0.5
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Hasnain Afzal

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Re: If 12a + 3b = 1 and 7b – 2a = 9, what is the average (arithmetic mean)   [#permalink] 18 Aug 2018, 09:55
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