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MGMAT Flash Cards Question

Problem Solving — algebra and arithmetic (GMAT Focus Edition)
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Source: — Quantitative Reasoning |
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by papgust » Thu Feb 25, 2010 5:56 pm
S3 = 0

S4 = S4-1 - 10 = S3 - 10 = -10

S5 = S4 - 10 = -10 -10 = -20

Common difference b/w each term is -10 (d). The question asks for S25. Now we know S5.

S25 = S5 + n * d [n - number of terms b/w S5 and S25]

= -20 + (20 * -10)

= -20 -200 = -220.
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