Let me attempt to explain.

As Kailing has given 0.3 of her share to Jim, she is left with 0.7. Translate this 0.7 into 7U.

All the 3 children has 7U each in the end; totaling 21U

After receiving the money from Kailing, Jim gave 1/3 to his brother. Jim is left with 2/3 which is 7U, hence 1/3 of Jim's money is 3.5U.

Now, since the bro received 3.5U from Jim, he would have 3.5U at first (remember he has 7U in the end)

No. of units Kailing and Jim have at first

21U - 3.5U = 17.5U

17.5U -> 63

1U -> 3.6

Kailing has 10U at first, hence she has $36.

Hope this solution make sense.