There's a question about calculation of DNI that I totally confused. Could anybody help me to explain them?

Simple trust has the following items of income and expenses: rents 25k, dividend 50k, tax-exempt interest 25k, LT capital gain 15k, tax and expenses directly attibutable to rent 5k, trustee's commission allocable to income account 2600, trustees's commission allocable to principal account 1300 depreciation 5k. comput the DNI.

The answer is 91100

DIV         500 00

Rent        25000

LTCG       15 000

Less:

Rent expense   (5000)

Commission allocable to imcome     (1950)

Commission allocable to principal    (975)

personal exemption      (300)

taxable imcome           81775

add back personal exemption     300

add tax exempt interest        2402 5

substract LTCG       15000

DNI     91100

I do not know why the commission allocable to principal is 975 but not 1300 and how to get the 975. Also I don't know how to calculate the 24025 of tax-exempt interest. 

Here is the link with the explanations   http://books.google.com/books?id=jUwd5ta1tz0C&pg=PA125&a mp;dq=attributable+to+rents.+5000.+Trustee's+commissions+all ocable+to+income+account&hl=en&sa=X&ei=0G0kT6_gA 7LfsQLcldyMAg&ved=0CDcQ6AEwAQ#v=onepage&q=attributab le%20to%20rents.%205000.%20Trustee's%20commissions%20allocab le%20to%20income%20account&f=false