Question 575139
This is the Fibonacci sequence. The first two terms in the sequence are, by definition, 0 and 1. After that, each added term is the sum of the two terms that immediately precede it. An extension of this sequence is:
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0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, ...
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So the answer to this problem is 21 because it is the sum of the 8 and the 13 that are the two terms that come immediately before it.
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For an explanation of this sequence, you can go to http://en.wikipedia.org/wiki/Fibonacci_number.
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Hopefully this helps you to understand the Fibonacci number pattern and how we determined the answer to this problem. 
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