Bonsoir
A= 2/15 + 7/5
= 2/15 + (7x3) / (5x3)
= 2/15 + 21/15
= 23/15
= 3x9 / 3x5
= 9/5
B= 5/7 - 31/21
= (5x3) / (7x3) - 31/21
= 15/21 - 31/21
= -16/21
C= 3/7 + 2/5
= (3x5) / (7x5) + (2x7) / (5x7)
= 15/35 + 14/35
= 29/35
D= 7/18 - 1/12
= (7x2) / (18x2) - (1x3) / (12x3)
= 14/36 - 3/36
= 11/36