设原来有酒x公升，水40%x/60%=2/3x公升x+15=(x+2/3x+15)x75%1/4x=15/4x=15所以原来有酒15公升
设原来有酒x公升，有水x*40%/60%=2/3x公升x+15=(x+2/3x+15)*75%x+15=5/4x+45/41/4x=15/4x=15原来有酒15公升
设原有x公升酒。则 （x+15）÷（x÷0.6+15）=0.75解得x=15
