Salut! :)
a)
51/14 : 17/7 × 2/3 =
51/14 × 7/17 × 2/3=
714/714 = 1
b)
15/8 × 4/9 : 25/3 =
60/72 × 3/25 =
180/1800 = 1/10
c)
27/8 : 5/4 : 3/5 =
27/8 × 4/5 × 5/3 =
540/120