16. From city $A$ to city $B$, one can travel by one of three types of transport, and from city $B$ to city $C$ - by one of four types of transport. In how many ways can one travel from city $A$ to city $C$, visiting city $B$ along the way?
16. From city $A$ to city $B$, there are three ways to travel, after which from city $B$ to city $C$, there are four ways. By the rule of product, from city $A$ to city $C$, there are $3 \cdot 4=12$ ways to travel.