(х - 3 13/21) + 2 10/21 = 7 2/21
х - 3 13/21 = 7 2/21 - 2 10/21 = 5 + (2/21-10/21) = 4 + (21/21+2/21-10/21) = 4 (21+2-10)/21 = 4 13/21
х = 4 13/21 + 3 13/21 = 7 26/21 = 5 5/21
х - 2 1/5 = 5 2/5
х = 5 2/5 + 2 1/5 = 7 3/5
8 - х = 3 5/21
х = 8 - 3 5/21 = 5 - 5/21 = 4 + (21/21-5/21) = 4 16/21