In a taxi, a passenger can sit in the front and three passengers can sit in the back. In how many ways can four passengers sit in a taxi if one of these passengers wants to sit by the window?
The passenger who wants to sit by the window has three possible seats, the next can sit in any free seat, thus having three possible seats; the next has two possible seats, and the last one has no choice. We conclude that the number of these ways of sitting is $3 \times 3 \times 2=18$.