To explain by mathematics is to find the equivalence relation. There are many, just to give an example:
1, A/R and A/P balance, that is, A/R = A/P.
10×3=30
Or (10- 1)×3+5-2=30.
2. Display the paid-in balance of the boss.
( 10- 1)×3-2=30-5
3. According to the balance of passenger expenditure.
( 10- 1)×3=30-5+2
Or 10×3-3=30-5+2.
4, according to "buddy" corruption.
( 10- 1)×3-(30-5)=2
The direct answer to your last question is the second one above.
That is, 27-2=30-5, (27+2 is meaningless)
mean
The meaning on the left is: passengers pay -man corruption = preferential price,
Left: Hotel receivables-preferential amount = preferential price.
...
I hope it helps you!