12÷(1- 1 7 )=12÷ 6 7 =14(升)12÷(1- 3 4 )=12÷ 3 4 =16(升)丙原有:14-16× 1 4 =14-4=10(升)甲原有:(12-14× 1 7 )÷(1- 1 3 )=(12-2)÷ 2 3 =10÷ 2 3 =15(升)乙原有:12×3-15-10=36-15-10=11(升)答:甲桶原有15升,乙桶原有11升,丙桶原有10升.