(1)23÷50=0.46(元),
(29.2-60×0.46)÷(80-60),
=1.6÷20,
=0.08(元);
答:1~60度、61~100度这两段的电费分别是每度0.46元和0.08元.
(2)60×0.46+(90-60)×0.08,
=27.6+2.4,
=30(元);
答:若该户居民6月份用电量是90度,应交电费30元.
(3)48.12-60×0.46,
=48.2-27.6,
=20.6,
20.6÷0.08=257.5(度),
257.5+60=317.5(度);
答:若该户居民7月份缴电费是48.12元,该户7月份用电317.5度.