Question

There are 5 different roads from city A to city B and 3 different roads from city B to city C. In how many ways can someone go from city A to city C passing by city B?

Answer 1

When someone goes from city A to city C passing by city B, at fist he chooses one of the 5 different roads from city A to city B - these are 5 variants. Then for each vatiant he chooses one of 3 different roads from city B to city C, as a result we obtain $5 * 3 = 15$ different variants goes from city A to city C passing by city B.