это дроби: 5/7 + 1/4, 11/18 - 4/9

5/7 + 1/4 = 5*4/7*4 + 1*7/4*7 = 20/28 + 7/28 = 20+7/28 = 27/28

11/18 - 4/9 = 11/18 - 4*2/9*2 = 11/18 - 8/18 = 3/18 = 3/3*6 = 1/6