答:
设加入水的体积是V 毫升,溶质质量 = 60x2x15% ;溶液质量 = 120+V,所以:
60x2x15%/[(120+V)]X100% = 0.5%
V =[ 60X2X0.15% - 2X60X0.5%]/5% = 3480 (毫升)