Question

There is an auditorium with $35$ rows of seats. There are $20$ seats in the first row, $22$ seats in the second row, $24$ seats in the third row and so on. Find the number of seats in the twenty-third row.

Answer 1

Seats are to be arranged as $20$ in the first row, $22$ in the second row, $24$ in the third row, and so on.
$\therefore$ Here $a=20,d=22-20=2$
We have to find $a_{23}$.
We know that
$a_n=a(n-1)d$
$a_{23}=20+(23-1)\times 2$
$=20+22\times 2$
$=20+44$
$=64$