KEAM 2010 Math Paper

Let a be first term and d be common difference of AP. Then, according to question
(11th+12th+13th) term =141
⇒‌3a+33d=141....(i)
and (21th+22th+23th) term =261
⇒‌3a+63d=261.....(ii)
Solving Eqs. (i) and (ii), we get
a=3,d=4
∴ First term =3