1) 1/2 + 1/3 = (3+2)/6 = 5/6
2) 1/4 - 1/5 = (5-4)/20 = 1/20
3) 1/6 * 1/12 = 1/72
4) 2/3 - 4/27 = (18-4)/27 = 14/27
5) 7/9 + 5/12 = (28+15)/36 = 43/36 = 1 7/36
6) 11/12 - 5/18 = (33-10)/36 = 23/36
7) 17/20 + 11/15 = (51+44)/60 = 95/60 = 1 35/60 = 1 7/12