Question

A meeting Hall has 20 seats in the first row,20 seats in the second , 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

Sol. The no. of seats in each row are as follows 20, 24, 28, .........

The no. of seats in each row form an A.P. with

First term (a) = 20

Difference between the no. of seats in two successive rows (d) = 4

Total no. of rows (n) = 30

Total no. of seats in 30 rows (S30) = ?

Sn = n/2[2a + (n – 1) d]

∴  S30 = 30/2 [2 (20) + (30 – 1) 4]

∴  S30 = 15 [40 + 116]

∴  S30 = 15 (156)

∴  S30 = 2340

∴ Total no. of seats in the meeting hall is 2340.