aidez moi pour l'exercice 2 svp?

a) (1/3 - 1/5) : 2/5
= (1 x 5 / 3 x 5 - 1 x 3 / 5 x 3) : 2/5
= (5/15 - 3/15) : 2/5
= 2/15 x 5/2
= 10/30
= 10 x 1 / 10 x 3
= 1/3

b) 4/7 - 1/7 x 5/3
= 4/7 - 5/21
= 4 x 3 / 7 x 3 - 5/21
= 12/21 - 5/21
= 7/21
= 7 x 1 / 7 x 3
= 1/3

c) 7/8 - 4/15 x 25/15
= 7/8 - 100/225
= 7 x 225 / 8 x 225 - 100 x 8 / 225 x 8
= 1575/1800 - 800/1800
= 775/800
= 25 x 31 / 25 x 32
= 31/32