a. 1/13 + 1/39= 1×3/13×3= 3/39 + 1/39 = 4/39
b 1/13 - 1/39= 1×3/13×3= 3/39 -1/39= 2/39
c. 1/17 + 1/51 = 1×3/17×3= 3/51 + 1/51= 4/51
d. 1/17 - 1/51 = 1×3/17×3= 3/51 - 1/51= 2/51
e. 9/15 + 3/5= 3×3/5×3= 9/15 + 9/15= 18/15
f. 9/15 - 3/5= 3×3/5×3= 9/15 - 9/15= 0