1.3/8+5/24=9/24 (8*3)+5/24=14/24=7/12 2.3/5-1/7=15/35-5/35 (5*7; 7*5)=10/35=2/7 3.9/17-19/68=32/68-19/68(17*4)=13/68 4.4/15-11/60=16/60-11/60 (15*4)=5/60=1/12