Question

An auditorium has 20 rows with 10 seats in the first row, 12 in the second row, 14 in the third row, and so forth. How many seats are in the auditorium?

Answer 1

This is fairly easy to get, you just need some logical thinking.

Understand that the auditorium has 20 rows, the number of seats increases by 2 every row and row 1 has 10 seats.

Row 1 has 10

Row 2 has 12 and so forth.

10 + 12 + 14 + 16 + 18 + 20 + 22 + 24 + 26 + 28 + 30 +32 +34 +36 + 38 + 40 +42 + 44 + 48 + 50

Added up is 584.

The auditorium has 584 seats

Answer 2

As you can see,
They are in A.P
a=10
n=20
d=12-10=2
S20=
$n \div 2 \times (2a + (n - 1 )d)$
So if you will insert the values,
You will get the ans580