Answer
$2869$ seats
Work Step by Step
Sum of the an arithmetic series can be calculated by using formula $S_n=\dfrac{n}{2}(2a_1+(n-1)d)$
Our aim is to determine a formula for the general term of the arithmetic sequence $a_n$.
Such as: $a_n=a_1+(n-1)d$
For $n=38$
$s_{38}=19(a_1+a_{38})$ ...(1)
Now, $a_n=a_1+(n-1)d$
$\implies a_{38}=a_1+(37)(3)=20+(37)(3)=131$
From equation (1), we have
$s_{38}=19(20+131)=2869$
