Bonjour
a. 2/15 + 7/5
= 2/15 + (7 x 3) / (5 x 3)
= 2/15 + 21/15
= 27/15
b. 5/7 - 31/21
= (5 x 3) / (7 x 3) - 31/21
= 15/21 - 31/21
= - 16/21
c. 3/7 + 2/5
= (3 x 5) / (7 x 5) + (2 x 7) / (5 x 7)
= 15/35 + 14/35
= 29/35
d. 7/18 - 1/12
= (7 x 2) / (18 x 2) - (1 x 3) / (12 x 3)
= 14/36 - 3/36
= 11/36.