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Learn how to solve the problem of subsection charging through the math problems of grade one in junior high school.
Let the first paragraph be a yuan per cubic meter and the second paragraph be b yuan per cubic meter. The fees to be paid are:

Jia Xiaoming 15a+(20- 15)b=30.

Jia Xiaohua 15a+(30- 15)b=54.

Combine the above two formulas to get b=2.4 and a= 1.2.

So Xiaoli's family has to pay a fee.

15a+(35-15) b =15×1.2+20× 2.4 = 66 (yuan)