Arithmetic series

An auditorium where COVID-19 Vaccination occurs has 20seats on the first row,24 seats on the second row and 28 seats on the third row and so on and has 30 rows of seats.How many seats are there in the auditorium?

Given: a1=_____,d=____,n=_____
Asked: How many seats are there
in auditorium?
Formula:
solution:​