There are three stations $A,B$ and $C$, five routes for going from station $A$ to station $B$ and four routes for going from station $B$ to station $C$.
Find the number of different ways through which a person can go from station $A$ to $C$ via $B$

Since there are five routes for going from station A to station B
So number of ways of going from A to B is $5$
And there are four routes for going from B to C, so the number of ways of going from B to C is $4$
Thus using multiplication rule, number of ways of going from A to C via B is, $5\times 4=20$