Х+4/15=2/3+2/5
Х=10/15+6/15-4/15=12/15=4/5
Х=4/5
(4/5 -Х)+13/20=25/30
4/5+13/20 -25/30 = Х
48/60 +39/60 -50/60=Х
Х=37/60
У- 5/20 = 5/8 - 3/10
У= 5/8+5/20-3/10
У= 25/40 + 10/40 -12/40 =23/40
2/3 -(7/9 -а)=1/3
2/3 - 7/9 +а = 1/3
а=1/3-2/3+7/9=3/9 -6/9 +7/9= 4/9