如图,C、D是线段AB上两点,AC、CD、DB的长度比为1:2:3,又M为AC的中点,DN：NB=2：3,已知AB=30

AC=AB/(1+2+3)=AB/6
MC=AB/12
CD=2AB/(1+2+3)=AB/3
DB=3AB/(1+2+3)=AB/2
DN=2DB/(2+3)=2DB/5=AB/5
MN=MC+CD+DN=AB/12+AB/3+AB/5=AB*37/60=18.5CM