Exercice 71 urgent svp

bonjour

a = 24/18 x 5 - 1/2 x 20/3
a = 120/18 - 20/6
a = 120/18 - 60/18
a = 60/18 = 10/3

b = 7 - 1/15 x 5
b = 7 - 5/15
b = 105/15 - 5/15
b = 100/15 =  20/3

c = 3/21 x ( 13/5 - 18/15) x 3/2
c = 3/21 x ( 38/15 - 18/15) x 3/2
c = 3/21 x 20/15 x 3/2
c = 180/630
c = 2/7