30. A theatre has 40 rows with 30 seats in the first row, 33 in the second row, 36 in the third
row and so on. How many seats are there in the theatre?

No. of seats in 1st row = 30
No. of seats in 2nd row = 33
No. of seats in 3rd row = 36

This forms an A.P.,
30,33,36........
First term = a = 30, Common Difference = d = (33-30 = 3) = 3

S_n = n/2 {2a + (n-1) d}

S₄₀ = 20 {60 + 117)
S₄₀ = 20 x 177
S₄₀ = 3540 

No. of seats in the theatre = 3540 seats