设乙取X千克,甲取2x千克,丙取;20-3X千克.
10(2x)+12x+(20-3x)15=20x12.4
20x+12x+300-45x=248
13x=52
x=4
答,乙取4千克,甲取2x4=8千克,丙取;20-3X4=8千克.