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In a sequence of numbers, the first number is 3, the second

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In a sequence of numbers, the first number is 3, the second [#permalink] New post 21 Aug 2003, 04:28
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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 [#permalink] New post 21 Aug 2003, 04:46
Essentially it becomes that subsequent number is 2 times the previous number..

So my answer is D , 32x
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 [#permalink] New post 21 Aug 2003, 05:13
another vote for (D)
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 [#permalink] New post 21 Aug 2003, 05:15
You're both correct

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 [#permalink] New post 24 Jul 2004, 08:08
Can anyone prove this ?
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 [#permalink] New post 24 Jul 2004, 08:13
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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 [#permalink] New post 24 Jul 2004, 09:22
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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 [#permalink] New post 24 Jul 2004, 10:29
Oops, didn't read the question properly :oops:

Thanks for your explanations!
  [#permalink] 24 Jul 2004, 10:29
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In a sequence of numbers, the first number is 3, the second

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