If 4A = 5B and 6B = 7C, then A : C = ?

Here,

4A = 5B ⇒ A : B = 5 : 4

6B = 7C ⇒ B : C = 7 : 6

So, by balancing ratio 

A : C = (7 × 5) : (6 × 4) = 35 : 24