I. Charge of electricity by stages: Example 1 (Wuhan) The basic electricity price of a residential area in a certain city is 0.4 yuan per kWh. If the monthly electricity consumption exceeds one kWh, the excess will be charged at 70% of the basic electricity price. (1) A household used 84 kWh of electricity in May, and * * * paid 30.72 yuan for electricity, so find A; (2) If the average household electricity bill in June is 0.36 yuan per kWh, how many kWh of electricity will be used in June? How much electricity should I pay? Solution: Assume that the monthly electricity consumption of this household is X degrees, and the electricity bill paid is Y yuan. According to the meaning of the question, the function relationship can be constructed by stages: when x≤a, y = 0.4a as x>a, y=0.4a+0.4×70%(x-a)( 1). Because the electricity consumption in May was 84 kwh, * * * paid 30.72 yuan, and its value was first put into (1) for judgment. Because 0.4× 84 = 33.6 >: 30.72, the electricity consumption in May exceeds one degree, which should satisfy the analytical formula (2). So 30.72=0.4a+0.4×70%(84-a), the solution is a=60. (2) Because 0.36