Bonjour
a. 4/7 = (4 x 5) / (7 x 5) = 20/35
b. 8/5 = (8 x 9) / (5 x 9) = 72/45
c. 21/56 = (21 : 7) / (56 : 7) = 3/8
d. 12/18 = (12 : 6) / (18 : 6) = 2/3.