An overnight express company must include nine cities on its route. How many different routes are possible, as

Question

An overnight express company must include nine cities on its route. How many different routes are possible, assuming that it does not matter in which order the cities are included in the routing

Answer 1

Think about it this way...

the first city can have eight possible routes to the eight other cities
the second city would have seven additional routes (because we've already counted the route between it and the first city)
the third city would have six additional routes
and so on...