VTMO 2026 - Grade 10 - Semifinal - SET 1 - Self-study Question 14

A finite arithmetic progression $a_1,a_2,\ldots,a_{6n+1}$ satisfies $a_1+a_3+a_5+\cdots+a_{6n+1}=320$ and $a_1+a_4+a_7+\cdots+a_{6n+1}=224$. Find $a_1+a_2+\cdots+a_{6n+1}$.