(1)0.55x+(100-x)×0.35=0.2x+35;
(2)当x=40时,0.2x+35=43元;
(3)当y=50时,0.2x+35=50,解得x=75,
∴100-x=25千瓦时.
答:(1)该居民户这个月应缴纳电费为0.2x+35元;
(2)当x=40时,求应缴纳电费为43元;
(3)若缴纳电费为50元,求谷时段用电25千瓦时.
(1)0.55x+(100-x)×0.35=0.2x+35;
(2)当x=40时,0.2x+35=43元;
(3)当y=50时,0.2x+35=50,解得x=75,
∴100-x=25千瓦时.
答:(1)该居民户这个月应缴纳电费为0.2x+35元;
(2)当x=40时,求应缴纳电费为43元;
(3)若缴纳电费为50元,求谷时段用电25千瓦时.