1-1/3. 1-3/7. 1-8/8. 1-3/27

представим 1 как 3/3, 
1 - 1/3 = 3/3 - 1/3 = 2/3
аналогично:
1 - 3/7 = 7/7 - 3/7 = 4/7
1 - 8/8 = 8/8 - 8/8 = 0
1 - 3/27 = 27/27 - 3/27 = 24/27