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Difficult Sequence (Posted on 2003-10-18) Difficulty: 5 of 5
What is the next number in the sequence? 21,45,66,102,115,171,?

See The Solution Submitted by Freddy Grants    
Rating: 3.6667 (9 votes)

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An alternate approach | Comment 7 of 19 |
It is possible that this sequence is two sequences intertwined. Consider every other term: 21, 66, 115, X. The short sequence 21, 66, 115 can be interpreted as a quadratic sequence, which makes the next term 168. Similarly the term after 168 is 252. 252 comes from treating 45, 102, 171 as a quadratic sequence.
Edited on October 21, 2003, 1:53 pm
  Posted by Brian Smith on 2003-10-21 13:22:16
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