Réponse :
b / 9/7-18/21 = 9x3/7x3 - 18/21 =27/21 -18/21 = 9/21 = 3x3/7x3 = 3/7
c/ 2/5 +7/20 = 2x4/5x4 + 7/20 = 8/20 + 7/20 = 15/20 = 5x3/5x4 = 3/4
d/ 2/5 - 20/25 = 2x5 / 5x5 - 20/25 = 20/25 - 20/25 =0
