1/2 + 2/5
1/2- 2/5
1/4+1/6
9/10- 5/6

1/2+2/5=5/10+4/10=9/10
1/2-2/5=5/10-4/10=1/10
1/4+1/6=3/12+2/12=5/12
9/10-5/6=54/60-50/60=4/60=1/15