a) 3/5 - (-7/10) - (13/-20)
= 3/5 + 7/10 + 13/20
= 12/20 + 14/20 + 13/20
= 39/20
b) 3/4 + (-1/3) - 5/18
= 3/4 - 1/3 - 5/18
= 27/36 - 12/36 - 10/36
= 5/36
c) 3/14 - (5/-8) + (-1/2)
= 3/14 + 5/8 - 1/2
= 12/56 + 35/56 - 28/56
= 19/56
d) 1/2 + (1/-3) + 1/4 - (-1/6)
= 1/2 - 1/3 + 1/4 + 1/6
= 12/24 - 8/24 + 6/24 + 4/24
= 14/24
= 7/12