Mathematics

An arithmetic progression has a constant step between terms, and there's a direct formula that links the sum of the first $n$ terms to the first term $a$, the number of terms $n$, and the common difference $d$. Rather than adding all 20 terms individually, use the sum formula $S_n = \frac{n}{2}(2a + (n-1)d)$ to set up one equation with one unknown—then solve for $a$.

An arithmetic progression has a constant step between terms, and there's a direct formula that links the sum of the first $n$ terms to the first term $a$, the number of terms $n$, and the common difference $d$. Rather than adding all 20 terms individually, use the sum formula $S_n = \frac{n}{2}(2a + (n-1)d)$ to set up one equation with one unknown—then solve for $a$.