Bonjour
C = 2/7 - 15/7 : 5/4
C = 2/7 - 15/7 x 4/5
D = (2 x 5) / (7 x 5) - 60/35
D = 10/35 - 60/35
D = - 50/35
D = (5 x - 10) / (5 x 7)
D = - 10/7.
D = (4/3 + 3/10) : (5/2 - 2/5)
D = [(4 x 10) / (3 x 10) + (3 x 3) / (10 x 3)] : [(5 x 5) / (2 x 5) - (2 x 2) / (5 x 2)]
D = (40/30 + 9/30) : (25/10 - 4/10)
D = 49/30 : 21/10
D = 49/30 x 10/21
D = 490/630
D = (70 x 7) / (70 x 9)
D = 7/9
