Total amount of oil in drum= 80 litres

Total amount of oil in 4 drums of 10 litre = 40 litre.

Total amount of oil in 6 drums of 5 litre = 30 litre.

Therefore, total oil filled = 30 litre + 40 litre= 70 litre.

Oil left = 80 litre – 70 litre = 10 litre.

This can be filled in 2 litre drums.

Therefore, number of drums of 2 litre = 10 ÷ 2 = 5 drums.