40(x+0.03)+60(x-0.25)=42.73

Solution: x=0.5653

0.5653+0.03=0.5953 (Yuan)

0.5653-0.25=0.3 153 (yuan)

A: The flat electricity price paid by Xiaoming's family this month is 0.5953 yuan per kWh, and the valley electricity price is 0.3 153 yuan per kWh.

(2)(40+60)×0.5653=56.53 (yuan)

56.53-42.73= 13.8 (yuan)

Answer: If the time-of-use electricity price is not used for settlement, Xiaoming's family will pay more electricity in May 13.8 yuan.