Question 978611
This is an arithmetic series
The first row has 30 seats, and the 13th row is 12 rows behind.

N(13)=20+(13-1)*5
N=20*60=80

Sum=(1/2) {first row + last row}* n rows
There are 20 rows, and by the above formula, row 20 has 20+(19*5)=115 seats.
sum=(1/2)*20 (20+115)=1350 seats
20+25+30+35+40+45+50+55+60+65+70+75+80+85+90+95+100+105+110+115 ;;; 20 rows

adding the ones ending in 0, and get 650
adding together the ones ending in 5=700
sum is 1350