13/15-1/3= вычеслить дробь
8/21-2/7=
14/27-4/9=
19/28-3/14=
1/2+1/3=
2/7-1/9=
2/3+5/8=
4/5-5/9=

13/15 - 5/15 = 8/15
8/21 - 6/21 = 2/21
14/27 - 12/27 = 2/27
 19/28 - 6/28 = 13/28
3/6 + 2/6 = 5/6
18/63 - 7/63 = 11/63
16/24 + 15/24 = 31/24 = 1 7/24
36/45 - 25/45 = 11/45