Area that needs to be painted=1600 sq.ft.
1 gallon of paint covers 250 sq.ft.
Total area for which paint needs to be bought=1600-250 sq.ft=1350 sq.ft. [Since, Mr. Arnold already has 1 gallon of paint]
Total paint that needs to be bought=1350/250=5.4 gallons
Alternatively,
Total paint required=(Area to be painted/Coverage of 1 gallon of paint)-1 [Since, Mr. Arnold already has 1 gallon of paint]
Total paint required= (1600/250)-1 gallons
=6.4-1 gallons=5.4 gallons of paint is required