1/2+1/3 1/4+2/5 2/3+1/4 3/4+2/5 5/7-1/6 3/4-1/3
1/2+1/3=3/6+2/6=5/6
1/4+2/5=5/20+8/20=13/20
2/3+1/4=8/12+2/12=11/12
3/4+2/5=15/20+8/20=23/20=1 3/20
5/7-1/6=30/42-7/42=23/42
3/4-1/3=9/12-4/12=5/12