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a = 12/5 - 21/45  = 108/45 - 21/45 = 87/45 = 29/15

b = (2/3 -3) : 1/9
b = (2/3 - 9/3) : 1/9
b = - 7/3 x 9 = -63/3 = -21

c = (2/3 - 3/2) : 7/11
c = ( 4/6 - 9/6) : 7/11
c = - 5/6 x 11/7 = - 55/42

d = 1/3 +5/6 x 2/3
d = 1/3 +10/18
d = 6/18 +10/18 = 16/18 = 8/9

e = 2 - 5/2 x 4/15
e = 2 - 20/30
e ) 2 - 3/3
e  = 6/3 - 3/3 = 3/3 = 1 

f = ( 15/8 - 1/4) : ( 3/4 + 15/12)
f = ( 15/8 - 2/8) : ( 9/12 + 15/12)
f = 13/8  : 24/12
f = 13/8 x 12/24 
f =13/8 x 1/12 = 13/96

Said mange 1/4, reste 3/4
Agathe mange : 2/3 x 3/4 = 6/12 ou 1/2

à tous les 2 ils ont mangé : 1/4 + 1/2 = 1/4 + 2/4 = 3/4 il reste 1/4

1/4 = 5 gâteaux
4/4 =  20 gâteaux