5/11 - 3/11 + 7/11 = 9/11
9 13/19 + (8 18/19 - 3 15/19) = 9 + 8 - 3 + 13/19 - 15/19 = 14/19
10 4/21 - (4 10/21 + 3 19/21) = 10 - 4 - 3 + 4/21 - 10/21 - 19/21 = 3/21 = 1/7
5/11 - 3/11 + 7/11 = 9/11
9 13/19 + (8 18/19 - 3 15/19) = 9 + 8 - 3 + 13/19 - 15/19 = 14/19
10 4/21 - (4 10/21 + 3 19/21) = 10 - 4 - 3 + 4/21 - 10/21 - 19/21 = 3/21 = 1/7