Method 1: Solution: Let Mr. Li's family use X-kilowatt electricity during the peak hours in March this year, and then use (70-X) kilowatt electricity during the peak hours. Judging from the meaning of the question.

0.55X + 0.30(70-X)=0.52 x 70 - 5.40

Solve the equation and get

X=40 (kWh)

Method 2: Solution: (70x0.52-5.40)-(70x0.30) ÷ (0.55-0.30) = 40 (kWh).