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January 31, 2015

January 31, 2015

Posted by **Melissa** on Saturday, December 17, 2011 at 12:46am.

- Math -
**Steve**, Saturday, December 17, 2011 at 6:01amT13 = a+12d = 61

T20 = a+19d = 82

so,

T20-T13 = 7d = 21

d = 3

a = 25

S25 = 25/2 (25 + 25+24*3)

= 25/2 (122) = 25*61 = 1525

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