Answer:
As Boing is much better in the cost per seat It will purchase from Boing and only to fullfil the requirement
1 airbus   320  seat  $   200
18 boing     4500  seat  $  2,250
2 loockheed   550 seat  $   400
Total        5,370 seat    $  2, 850
Explanation:
Airbus    320 seat  $ 200M
Boeing   250 seat  $ 125 M
Lockheed 275 seat  $ 200 M
We need to get 4,480 seats with a budget of 2,900 millions plus the composition requirement of twice as many Lockheed as Airbus:
Those we will purchase an Airbus and two Lockheed to get the requirement of twice as many Lockheed as Airbus
The rest of the budget will be expended on Boeing