MGMAT Flash Cards Question

[red1050]

I don't understand the following equation and answer from the MGMAT flash cards. Can someone elaborate for me?

Sn = Sn-1-10 and S3 = 0

Thanks.

[papgust]

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.