2/3 + 1 4/5 = 2/3 + 9/5 = 10/15 + 27/15 = 37/15 = 2 7/15
11/15 - 2 3/5 = 11/15 - 13/5 = 11/15 - 39/5 = - 28/5 = - 5,6
4 5/7 - 2 3/14 = 33/7 - 31/14 = 66/14 - 31/14 = 35/14 = 2,5
