Chapter 9 - Section 9.2 - Arithmetic Sequences - 9.2 Assess Your Understanding - Page 655: 67

$1647$

The number of seats form an arithmetic sequence, with $a_{1}=35, d=2, n=27$ The nth term is $a_{n}=a_{1}+(n-1)d$ so, $a_{27}=35+2(26)=87$ $S_{n}=\displaystyle \frac{n}{2}\left(a_{1}+a_{n}\right)$ $S_{27}=\displaystyle \frac{27}{2}(35+87)=\frac{27}{2}(122)=1647$