A childhood math problem: In arithmetic progression {{an}}, a 1+a2+a3= 12, a4+a5+a6=20.

a4+a5+a6=20

(a7+A8+a9)-(a4+a5+a6)=(a4+a5+a6)-(a 1+a2+a3)

=8

(a7+a8+a9)=28

S9 =(a 1+a2+a3)+(a4+a5+a6)+(a7+A8+a9)

= 12+20+28

=60