An oil tank has a capacity of 45 gallons. at the beginning of october it is 80% full. at the end of october it is 1/3 full. how many gallons of oil were used in october?

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