Answer :
18+20+22+24+26+28+30+32+34+36+38+40+42+44+46+48+50+52+54+56+58+60 = 858 seats
Since it’s 18 first then 20 and 22 you have to add two to 22 19 more times and then add 18 and 20
Since it’s 18 first then 20 and 22 you have to add two to 22 19 more times and then add 18 and 20