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Series calculation

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by rnaah » Tue Oct 11, 2011 4:04 am
The series, a1, a2, a3, ... is defined such that each term after the first is twice the previous one. If a5 - a2 = 12, what is the value of a1?

a) 4
b) 24/7
c) 2
d) 12/7
e) 6/7
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by Anurag@Gurome » Tue Oct 11, 2011 4:08 am
rnaah wrote:The series, a1, a2, a3, ... is defined such that each term after the first is twice the previous one. If a5 - a2 = 12, what is the value of a1?

a) 4
b) 24/7
c) 2
d) 12/7
e) 6/7
a2 = 2(a1),
a3 = 2(a2) = 4(a1),
a4 = 2(a3) = 8(a1),
a5 = 2(a4) = 16(a1)

Now, a5 - a2 = 12 implies 16(a1) - 2(a1) = 12 or 14(a1) = 12
So, a1 = 12/14 or 6/7

The correct answer is E.
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by GMATGuruNY » Tue Oct 11, 2011 4:53 am
I posted an alternate approach here:

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by Abhishek009 » Tue Oct 11, 2011 5:26 am
rnaah wrote:The series, a1, a2, a3, ... is defined such that each term after the first is twice the previous one. If a5 - a2 = 12, what is the value of a1?

a) 4
b) 24/7
c) 2
d) 12/7
e) 6/7
Terms No


1 a

2 2a

3 4a

4 8a

5 16a

... ...


Now , 5th term - 2nd term is 12

So , 16a - 2a = 12

Hence 14a = 12

Or, a = 12/14 => 6/7
Abhishek
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by GmatKiss » Tue Oct 11, 2011 6:10 am
form the series with the given options!

i tried with 2,4 and the 6/7

6/7 worked, IMO: E(1:00 min)
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