Ответ:

1)(7/15-1/6)+2/5=7/15-1/6=14/30-5/30=9/30=9/30+2/5=9/30+12/30=21/30

2)(3/8-1/9)+25/36=3/8-1/9=27/72-8/72=19/72=25/36+19/72=50/72+19/72=69/72

3)(8/9-5/6)+2/3=8/9-5/6=16/18-15/18=1/18=1/18+2/3=1/18+12/18=13/18

4)(7/8-13/20)+9/10=7/8-13/20=35/40-26/40=9/40=9/40+9/10=9/40+36/40=45/40=1 5/40

5)(7/18-1/12)+5/6=7/18-1/12=14/36-3/36=11/36=11/36+5/6=11/36+30/36=41/36

6)(3/4-8/15)+17/20=3/4-8/15=45/60-24/60=21/60=21/60+17/20=21/60+51/60=72/60=1 12/60