Dong Zhang went to the department store to buy two kinds of envelopes, ***30, of which 1 yuan 50 cents was used to buy an A-type envelope and 1 yuan 50 cents was used to buy a B-type envelope, and each B-type envelope was 2 points cheaper than the A-type envelope. What is the unit price of these two kinds of envelopes?

Solution: If the unit price of type A envelope is A, the unit price of type B envelope is a-2.

If you buy B-type A envelopes, you will buy 30-b-type B envelopes.

1 yuan.50 = 150.

ab= 150( 1)

(a-2)(30-b)= 150(2)

By (2)

30a-60-ab+2b= 150

Substitution (1)

30a- 150+2b=2 10

30a+2b=360

15a+b= 180

b= 180- 15a

Replace (1)

a( 180- 15a)= 150

Answer? - 12a+ 10=0

(a-6)？ =36- 10

a-6= √26

a=6 √26

A 1≈ 1 1 min, then the b-type envelope 1 1-2=9 points.

A2≈0.9, then 0.9-2=- 1. 1 of the type B envelope is irrelevant, so it is discarded.

The unit price of type A is 1 1, and that of type B is 9.