Solution: If barrel B has X kilograms of oil and barrel A has 2 kilograms of oil, then there are:

2X-2.5=X+2.5

D: X=5, so A 2X= 10.

Answer: barrel a oil originally contained 10 kg, while barrel b oil originally contained 5 kg.

Arithmetic method:

Solution: Let A be 1, and you get

Raw barrel oil: 2.5 ÷ (1+1/2) ÷ 2-1/2 =10 (kg);

Original barrel oil: 10÷2=5 (kg).

Answer: barrel a oil originally contained 10 kg, while barrel b oil originally contained 5 kg.