8/9 - 7/12 = (32/36) - (21/36) = 11/3617/20 - 5/12 = (51/60) - (25/60) = 26/60 = 13/307/15 - 3/20 = (28/60) - (9/60) = 19/6011/12 - 7/15 = (55/60) - (28/60) = 27/60 = 9/20

Ответы:

11/3613/3019/609/20