(2 3/5-1 7/10)+(1 1/2-7/20) = 6 1/20

1) 2 3/5-1 7/10 = 2 6/10 - 1 7/10= 9/10

2) 1 1/2-7/20 = 110/20 -7/20 = 5 3/20

3) 9/10 + 5 3/20 = 18/20 + 5 3/20 = 5 21/20 = 6 1/20