There are 2 major roads from city x to city y and 4 major roads from city y to city z. How many different trips can be made from city x to city z passing through city y?

2y (4z) = 8yz

each major road of city y you have 4 different routes so if there're 2 routes of city y then double the number of routes and you get 8 different roads to travel in from x to z through y