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Manager
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In a sequence of numbers, the first number is 3, the second [#permalink]
 21 Aug 2003, 05:28
In a sequence of numbers, the first number is 3, the second number is 7, and each subsequent number is the sum of all the preceding numbers in the list. If the 20th term is x, what is the 25th term?
A. 7x
B. 10x
C. 16x
D. 32x
E. 64x
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Manager
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Essentially it becomes that subsequent number is 2 times the previous number..
So my answer is D , 32x
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Brainless
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another vote for (D)
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Too much is not enough...
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Manager
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You're both correct
rich
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Senior Manager
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Can anyone prove this ?
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The sequence goes like this if you write down a couple of the few terms:
3,7,10,20,40,80,160... etc
So if 20th term = x
then 21 = 2x
22 = 4x
23 = 8x
24 = 16x
25 = 32x
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Manager
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Dookie wrote: Can anyone prove this ?
That's really easy:
sum of items from 1 to 19 is X, 20th item = X also, 21st = 2X, 22nd = 4X, ... item with index 20 + i is equal to 2^i*X.
25th item is equal to 2^5*x = 32*X.
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Senior Manager
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Oops, didn't read the question properly 
Thanks for your explanations!
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close
