bonjour
b = 7/5 - 3/5 x 4/21
b = 7/5 - 12/ 105
b = 147/105 - 12/105
b = 135/105 = 9/7
c = 13/7 - 3/7 : 3/5
c = 13/7 - 3/7 x 5/3
c = 13/7 - 15/21
c = 39/21 - 15/21
c = 24/21 = 8/7
d = 3/4 + 1/2 x ( 2/3 - 1)
d = 3/4 + 1/2 x ( 2/3 - 3/3)
d = 3/4 + 1/2 x - 1/3
d = 3/4 - 1/6
d = 9/12 - 2/12
d = 7/12