Question

A meeting hall has 20 seats in the first row, 24 deats in the second row, 28 seats in the third row, and so on and has in all 30 rows how many seats are there in the meeting hall ?

Answer 1

seats in each successive row is making an AP
therefore the AP is;
20,24,28..........
where, a=20
d=4
n[number of rows]= 30

Total seats,

Sn= n/2×[2a+(n-1)d]
=30/2×[2×20+(30-1)4]
= 15×[40+29×4]
=15×[40+116]
=15×156
=2340

So, there are total 2,340 seats