Let the executive class air tickets and economy class tickets sold be x and y.
Now, as the seating capacity of the aeroplane is 200, so x+y ≤ 200.
As 20 tickets for executive class are to be reserved, so we have x ≥ 20.
And as the number of tickets of economy class should be at least 4 times that of executive class y ≥ 4x.
Profit on sale of x tickets of 1st class and y tickets of economy class, Z = 1000x+ 600y.