a=25.9÷14.8=1.75(元)(32.97-1.75×15)÷(17.4-15)=(32.97-26.25),=6.72÷2.4,=2.8(元).即b=2.8元.15×1.75+(19.5-15)×2.8=26.25+4.5×2.8,=26.25+12.6,=38.85(元).即三月份交水费38.85元.故答案为:38.85.