If a/b = 3/4 and a/c = 5, what is b/c?

Answer 1

In the given, the common variable is a, so let's express each in terms of a
a/b = 3/4 ---> a = 3b/4
a/c = 5 ---> a = 5c

so obviously
3b/4 = 5c
3b = 20c , how about "un-crossmultiplying" ?
then b/c = 20/3

Answer 2

b/c = (a/c) / (a/b) = (5)/(3/4) = 20/3