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-first 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 $a_{21}$
We know that,
$a_n=a+(n-1)d$
$a_{21}=20+(21-1)\times 2$
$=20+20\times 2$
$=20+40$
$=60$