12+sqrt 5, is your answer, hoped this helped,
Capacity of the tank = 45 gallon
At the beginning of Oct., amount of oil in the tank = 45*0.8 = 36 gallons
At the end of Oct., amount of oil in the tank = 45*1/3 = 15 gallons
So, amount of oil used during Oct = (36-15) = 21 gallons
