Donnez le résultats en une fraction irréductible À= 3/7-2/7x21/8 B=5/4-2/3x9/16 C=(2+2/3)÷(4/5-2/3) D=(1-1/3-1/4)÷2/5 Aide moi svp Je ne comprends pas!

A = 3/7 - 2/7 x 21/8
A = 3/7 - 42/56
A = 3 x 8 / 7 x 8 - 42/56
A = 24/56 - 42/56
A = - 18/56
A = 2 x - 9 / 2 x 28
A = - 9/28

B = 5/4 - 2/3 x 9/16
B = 5/4 - 18/48
B = 5 x 12 / 4 x 12 - 18/48
B = 60/48 - 18/48
B = 42/48
B = 6 x 7 / 6 x 8
B = 7/8

C = (2 + 2/3) ÷ (4/5 - 2/3)
C = (2 x 3 / 1 x 3 + 2/3) : (4 x 3 / 5 x 3 - 2 x 5 / 3 x 5)
C = (6/3 + 2/3) : (12/15 - 10/15)
C = 8/3 : 2/15
C = 8/3 x 15/2
C = 120/6
C = 6 x 20 / 6 x 1
C = 20

D = (1 - 1/3 - 1/4) ÷ 2/5
D = (1 x 12 / 1 x 12 - 1 x 4 / 3 x 4 - 1 x 3 / 4 x 3) : 2/5
D = (12/12 - 4/12 - 3/12) : 2/5
D = (12/12 - 7/12) : 2/5
D = 5/12 : 2/5
D = 5/12 x 5/2
D = 25/24