#### Answer

4 drums

#### Work Step by Step

First, find total gallons of oil needed by multiplying gallons used per day by 30 days:
$2\times 30=60$
Next, find gallons of oil needed to be ordered by deducting the gallons on hand from the total gallons required. Gallons on hand is found by multiplying the gallons per barrel with the number of barrels that are on hand:
Gallons of oil on hand= $8\times5=40$
Gallons of oil that need to be ordered = $60-40=20$
Each drum contains 5 gallons. Number of drums needed to be ordered can be found by dividing the total gallons need to be ordered by gallons in each drum.
$20\div5=4$
Thus 4 drums need to be ordered