If it took 2.5 gallons of paint to cover 282 square feet of wall, how much paint is needed to paint an additional 423 square feet

Its 3.75 gallons because half of 282 is 141 and 141+282=423. So add half of 2.5 to the amount of gallons you have and you should get 3.75. Half of 2.5 is 1.25 and 1.25+2.50=1.75 (2.50 is the same thing as 2.5)