数学题:为节约能源,某物业公司按以下规定收取每月电费:用电不超过140度,按每度0.4

解：设四月份用电x度，电费0.5x元。根据题意，得0.43*140+0.57（x-140）=0.5x                                60.2+0.57x-79.8 =0.5x                                         0.57x -19.6= 0.5x                                                  0.07x=19.6                                                         x=280                                            0.5*280=140（元）答：四月份用电280度，应交电费140元。 收起