(1)249.5-242=7.5(立方米),
2.5x+5×(7.5-x)=25,
2.5x+37.5-5x=25,
37.5-2.5x+2.5x=25+2.5x,
37.5-25=25+2.5x-25,
12.5=2.5x,
12.5÷2.5=2.5x÷2.5,
x=5,
答:当用水不超过5立方米时享受优惠价2.5元/立方米;
(2 )261-249.5=11.5(立方米),
2.5×5+(11.5-5)×5,
=12.5+6.5×5,
=12.5+32.5,
=45(元),
答:小兰家4 月份应缴水费45元;
(3)(32.5-5×2.5)÷5+5,
=(32.5-12.5)÷5+5,
=20÷5+5,
=4+5,
=9(立方米),
答:2月份小兰家用水9立方米.