х +4/15 = 2/3+ 2/5
( 4/5 - х ) +13/20 = 25/30 
у - 5/20 = 5/8 - 3/10 
2/3 - (7/9 - а) =1/3

х=2/3+2/5-4/15х=12/15+4/15=16/15(4/5-х)=25/30-13/20=500-390/600=110/600=11/60х=4/5-11/60=48-11/60=37/60у=5/8-3/10+5/20у=23/40(7/9-а)=2/3-1/3(7/9-а)=1/3а=7/9-1/3а=7-3/9=4/9

х + 4/15 = 2/3 + 2/5х + 4/15 = 10/15 + 6/15х + 4/15 = 16/15х = 16/15 - 4/15х = 12/15 = 4/5         4/5 + 4/15 = 2/3 + 2/512/15 + 4/15 = 10/15 + 6/1516/15 = 16/15Ответ : х = 4/5(4/5 - х) + 13/20 = 25/304/5 - х = 25/30 - 13/204/5 - х = 50/60 - 39/604/5 - х = 11/60х = 4/5 - 11/60х = 48/60 - 11/60х = 37/60           (4/5 - 37/60) + 13/20 = 25/30(48/60 - 37/60) + 13/20 = 25/3011/60 + 13/20 = 25/3011/60 + 39/20 = 25/3050/60 = 25/3025/30 = 25/305/6 = 5/6Ответ: х = 37/60у - 5/20 = 5/8 - 3/10у - 5/20 = 25/40 - 12/40у - 5/20 = 13/40у = 13/40 + 5/20у = 13/40 + 10/40у = 23/40            23/40 - 5/20 = 5/8 - 3/1023/40 - 10/40 = 25/40 - 12/4013/40 = 13/40Ответ: у = 23/402/3 - (7/9 - а) = 1/37/9 - а = 2/3 - 1/37/9 - а = 1/3а = 7/9 - 1/3а = 7/9 - 3/9а = 4/9           2/3 - (7/9 - 4/9) = 1/32/3 - 3/9 = 1/36/9 - 3/9 = 1/33/9 - 1/31/3 = 1/3Ответ: а = 4/9