7. (6 points) There are two oil barrels, A and B, containing a total of 100 kilograms of oil. After transferring 15 kilograms of oil from barrel B to barrel A, the amount of oil in barrel A becomes 4 times that in barrel B. Therefore, originally, the amount of oil in barrel A was more than that in barrel B by $\qquad$ kilograms.
【Analysis】According to the problem, if we consider the total oil in barrels A and B as 5 parts, we can calculate how many kilograms each part is. After transferring 15 kilograms of oil from barrel B to barrel A, barrel A will account for 4 parts, and barrel B will account for 1 part. One part is $100 \div 5=20$ kilograms. We can then calculate the kilograms of oil in each barrel after the transfer. By subtracting 15 kilograms from the oil in barrel B and adding 15 kilograms to the oil in barrel A, we can find the original amounts of oil in barrels A and B. Finally, subtract the original amount of oil in barrel B from the original amount of oil in barrel A to get the answer.

【Solution】Solution: $100 \div (1+4)=20$ (kilograms)
Oil in barrel A after transfer: $4 \times 20=80$ (kilograms)
Oil in barrel B after transfer: $1 \times 20=20$ (kilograms)
Original oil in barrel A: $80-15=65$ (kilograms)
Original oil in barrel B: $20+15=35$ (kilograms)
The difference in oil between barrel A and barrel B: $65-35=30$ (kilograms)
Answer: Originally, barrel A had 30 kilograms more oil than barrel B.
Therefore, the answer is: 30.