设买了甲种纯净水x桶,乙种纯净水x÷ 75%桶
8x+(8-2)(x÷ 75%)=240
8x+6*4/3x=240
8x+8x=240
16x=240
x=15
15÷ 75%=20桶
答:买了甲种纯净水15桶,乙种纯净水20桶