Question

An auditorium has 21 rows of seats. The first row has 18 seats, and each succeeding row has two more seats than the previous row. How many seats are there in the auditorium?
1. 540
2. 567
3. 760
4. 798

Answer 1

798

Answer 2

This is an Arithmetic sequence (it increases by a set number each time)
Use:
\[S _{n} = \frac{ n(a₁ + a_{n}) }{ 2 }\]
Where n is the number of rows, a₁ is the number of seats in the first row, and an is the number of seats in the last row.