20_3/4 - (6 - 2_1/6) = 20_3/4 - (5_6/6 - 2_1/6) = 20_3/4 - 3_5/6 = 20_9/12 - 3_10/12 = 19_21/12 - 3_10/12 = 16_11/12
28_1/20 - (7 - 2_8/15) = 28_1/20 - (6_15/15 - 2_8/15) = 28_1/20 - 4_7/15 = 28_3/60 - 4_28/60 = 27_63/60 - 4_28/60 = 23_35/60 = 23_7/12
29_3/25 - (4 - 2_1/5) = 29_3/25 - (3_5/5 - 2_1/5) = 29_8/25 - 1_4/5 = 29_8/25 - 1_20/25 = 28_33/25 - 1_20/25 = 27_13/25