Track / Stage 4 / 74 of 340 #334 of 2000

6. Given the sequence $\left\{a_{n}\right\}$ is an arithmetic sequence, and $S_{5}=$ $28, S_{10}=36$. Then $S_{15}$ equals ( ).
(A) 80
(B) 40
(C) 24
(D) -48

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Official solution

6.C.

It is known that $S_{5}, S_{10}-S_{5}, S_{15}-S_{10}$ form an arithmetic sequence. It is easy to get $S_{15}=24$.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.