aider moi svp l'exercice 1 c pour demain

A = r + s - t
A = 3/7 + 5/6 - 14/15
A = 3 x 90 / 7 x 90 + 5 x 105 / 6 x 105 - 14 x 42 / 15 x 42
A = 270/630 + 525/630 - 588/630
A = 207/630
A = 9 x 23 / 9 x 70
A = 23/70

B = r x s x t
B = 3/7 x 5/6 x  14/15
B = 15/42 x 14/15
B = 210/630
B = 210 x 1 / 210 x 3
B = 1/3

C = r x t : s
C = 3/7 x 5/6 : 14/15
C = 15/42 x 15/14
C = 225/588
C = 3 x 75 / 3 x 196
C = 75/196

D = r x (s + t)
D = 3/5 x (7/6 + 14/15)
D = 3/5 x (7 x 10 / 6 x 10 + 14 x 4 / 15 x 4)
D = 3/5 x (70/60 + 56/60)
D = 3/5 x 126/60
D = 378/300
D = 6 x 63 / 6 x 50
D = 63/50