3.1/2:(1.4/15+3.9/10)

3.1/2:(1.4/15+3.9/10)=3.1/2:(2.8/30+11.7/30)=3.1/2:(14.7/30)=3.1/2*30/14.7=3.1*(15/14.7)=46.5/14.7