Before increasing the number of seats, price=20 so revenue=20*100,000=2,000,000.
After qty increased by 1000, you'll get the price as 19.9 for qty 101,000. Revenue =19.9*101,000=2,009,900.
Increase in Revenue =9900
At q = 100000, the price of 1 ticket would be p=20 from the given demand function.
When q = 101000, the price of 1 ticket would be p=19.9.
Since this would be a case of monopoly these new 1000 units would come under inframarginal units. The new price would be applicable for all previous units as well.
Increase in revenue due to extra seats = 19.9 * 1000 = 19900
Decrease in revenue due to decrease in price = (20-19.9)*100000 = 10000
Net increase in revenue = 19900-10000 = 9900