(13/15-3/5)+(17/30-1/10)=(13/15-9/15)+(17/30-3/30)=4/15+14/30=8/30+14/30=22/30=11/15
(5/9+2/3)-(11/18+1/6)=(5/9+6/9)-(11/18+3/18)=11/9-14/18=22/18-14/18=8/18
(5/7-3/14)-(25/28-9/14)=(10/14-3/14)-(25/28-18/28)=7/14-7/28=14/28-7/28=7/28
