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Medium level - sequence II

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by Brent@GMATPrepNow » Thu Mar 05, 2009 1:27 pm
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Please note that this is not an official GMAT question; it’s my attempt to create mid-range (500 to 650-level) GMAT-style questions for this forum.
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by truplayer256 » Thu Mar 05, 2009 2:32 pm
The above sequence is a geometric sequence. Therefore:

a1r^n-1 is equal to the nth term of the sequence. R is the common ratio of each of the preceding terms, which in this case is 2.

20th term=1(2)^19
21st term=1(2)^20
22nd term=1(2)^21

2^19+2^20+2^21=2^19(1+2+2^2)=7 * 2^19 B
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by Brent@GMATPrepNow » Thu Mar 05, 2009 2:41 pm
Nice work - the answer is B
Here's my solution as well.

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