设等质量都为m kg
2mol/L密度为A,4mol/L密度为B
c=n/v
n=3m/A+4m/B
v=m/A+m/B
c=(3m/A+4m/B)/(m/A+m/B)>3mol/L
希望有所帮助,密度符号不好打,用了不正式的符号,sorry.