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