Question

How to justify a/b x b/c = a/c?

Answer 1

well a/b * b/c = ab/bc, and since b/b= 1 you get 1a/c or just a/c :)

Answer 2

a/b times b/c is = ab/bc
which can be written as a/c times b/b
anything over itself is 1 so b/b is one
a/b times b/c = a/c