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In a certain list of numbers

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In a certain list of numbers [#permalink] New post 25 Nov 2012, 23:48
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In a certain list of numbers, the first number is 2, the second number is 3, and each succeeding number is the sum of all the numbers that precede it in the list. If h and k denote the 20th and 24th numbers in the list, respectively, what is the value of k/h ?

A. 5
B. 8
C. 10
D. 16
E. 20
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Re: In a certain list of numbers [#permalink] New post 26 Nov 2012, 00:13
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The trick here is to notice that:

1] This is actually a disguised Geometric Progression from the 3rd term onwards: 2, 3, 5, 10, 20, 40, 80...
2] Because the Geometric Progression starts from the 3rd term of the set, the nth term in the set is actually the (n-2)th term of the Geometric Progression part of the set

Now armed with these two observation, apply the formula for nth term of any Geometric Progression = b X r^(n-1); remember b is 5 and n will be the 18th and 22nd term respectively, NOT 20th and 24th because of the 2nd observation above.

h = 18th term of the GP = 5 X 2^17
k = 22nd term of the GP = 5 X 2^21

k/h = 2^4 = 16

Therefore D is the correct answer. ;)

Hope this helped.
_________________

The only ability the GMAT is an indicator of...is the ability to do well on the GMAT.

Re: In a certain list of numbers   [#permalink] 26 Nov 2012, 00:13
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