J'ai trouvé la 1 ere fraction apres j'y arrive pas merci de m'aider svp

Exercice 3
a) 7/5 = ../15 or 15 = 5*3 et 7*3 = 21 donc 7/5 = 21/15
7/5 = 14/... or 14 = 7*2 et 5*2=10 donc 7/5 = 14/10
7/5 = .../20 or 20 = 5*4 et 7*4=28 donc 7/5 = 28/20
7/5 = 42/... or 42 = 7*6 et 5*6=30 donc 42/30
réponse : 7/5= 21/15 = 14/10 = 28/20 = 42/30

b) 30/9 = 10*3/3*3 = 10/3 = 10*2/3*2 = 20/6 = 20*3/6*3 = 60/18 = 20*2/6*2 = 40/12
réponse : 30/9 = 10/3 = 20/6 = 60/18 = 40/12

c) 20/12 = 20*2/12*2 = 40/24 = 10*4/6*4 = 10/6 = 10*5/6*5 = 50/30 = 25*2/15*2 = 25/15
réponse : 20/12 = 40/24 = 10/6 = 50/30 = 25/15