The key to this problem is that the difference between them has not changed, because they have lent five extra-curricular books each, so this 5:4 ratio is obtained by simplifying the number ratio of their books, and 9:7 ratio is the same, so we should make the difference between these two ratios the same. The difference of 5:4 is 1 serving, and the difference of 9: 7 is 2, so I doubled 5:4 at the same time and became 10:8. At this time, the difference between the two is the same. Now we can see that the first child used to have ten copies, but now it has become nine copies. The difference in the middle is the five books he lent out, and so is the second one. It used to be eight books, and now it's seven books.

It should also be noted that after one copy is found to be five copies, the original ratio of 5:4 has now become 10:8. We found one copy according to 10:8, so we used 5× 10 and 5×8 to find the originals of two people respectively. Please adopt and collect me.