bonsoir
a = il ne manque pas des parenthèses ?
b = 7/6 - 30/5 = 35/30 - 180/30 = - 145/30 = - 29/6
c = 2/5 - 2/5 : 4/7 = 2/5 - 2/5 x 7/4 = 2/5 - 14/20 = 8/20 - 14/20 = - 6/20 = - 3/10
d = ( 21/28 - 32/28) : ( 35/10 - 18/10)
d = - 11/28 x 10/17
d = - 110/476 = - 55 /238