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The sum of the following series 1 + 6 + 7 ) 3 2 1 ( 9 2 2 2 + + + 9 ) 4 3 2 1 ( 12 2 2 2 2 + + + + 11 ) 5 ..... 2 1 ( 15 2 2 2 + + + + ..... up to 15 terms, is :

1. 7830
2. 7520
3. 7820
4. 7510

Correct answer: 7820

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