89
a = 1/5 - 40/60 = 12/60 - 40/60 = - 28/60 = - 7/ 15
b = ( 3/15 - 8/15) x 5/4 = - 5/15 x 5/4 = - 25/60 = - 5/12
c = - 4/7 x 14/5 + 5/6 = - 56/35 + 5/6 = -336/210 + 175/210 = - 161/210 = - 23/30
d = - 4/7 : ( 15/42 + 35/42) = - 4/7 : 50/42 = - 4/7 x 42/50 = - 168/294 = -4/7
pour le 88 , tu fais pareil
a c'est ( 3 - 2/3) : ( 4/3 x 7)
tu as plein d'exemples au dessus