(5/21+2/15)+2/21=7/21+2/15=(15*7+2*21)/(15*21)=(105+42)/315=147/315
(2/35+3/28)+1/28=2/35+4/28=(2*28+4*35)/(28*35)=(56+140)/980=196/980
(7/30+8/45)+(2/45+1/30)=7/30+8/45+2/45+1/30=8/30+10/45=(8*45+8*30)/(45*30)=(360+240)/1350=600/1350
(5/24+11/60)+(13/60+1/24)=5/24+11/60+13/60+1/24=6/24+24/60=1/4+4/10=1/4+2/5=(1*5+2*4)/20=13/20
(5/21+2/15)+2/21=7/21+2/15=(15*7+2*21)/(15*21)=(105+42)/315=147/315
(2/35+3/28)+1/28=2/35+4/28=(2*28+4*35)/(28*35)=(56+140)/980=196/980
(7/30+8/45)+(2/45+1/30)=7/30+8/45+2/45+1/30=8/30+10/45=(8*45+8*30)/(45*30)=(360+240)/1350=600/1350
(5/24+11/60)+(13/60+1/24)=5/24+11/60+13/60+1/24=6/24+24/60=1/4+4/10=1/4+2/5=(1*5+2*4)/20=13/20