Answer: 100 seats at 20th row
Explanation:
1st row & 2nd-row seats difference= 24-20 =4
2nd-row & 3rd row seats difference= 28-24 =4
Every row has extra 4 seats than the row before.
20th row there will be extra seats than the 1st row= (20*4)= 80 seats
So, 20th row seat= 20+80 = 100 seats