b.
9/7 - 18/21 = 9/7×3 - 18/21 = 9/21- 18/21 = 9/21
c.
2/5+7/20= 2/5×4 + 7/20 = 2/20 + 7/20 = 9/20
d.
2/5 - 20/25 = 2/5×5 - 20/25 = 2/25 - 20/25 = 18/25
voilà ! j'espère que cv t'aider
