15/20 - (х + 1/20)=4/20
(х + 1/20/) = 15/20 - 4/20
(х + 1/20/) = 11/20
х = 11/20 - 1/20
х = 10/20
15/20 - (10/20 + 1/20/) = 4/20
15/20 - (х + 1/20)=4/20
(х + 1/20/) = 15/20 - 4/20
(х + 1/20/) = 11/20
х = 11/20 - 1/20
х = 10/20
15/20 - (10/20 + 1/20/) = 4/20