Yann and Camille go to a restaurant.  If there are 10 items on the menu, and each orders one dish, how many different combinations of meals can Yann and Camille order if they refuse to order the same dish? (It does matter who orders what---Yann ordering chicken and Camille ordering fish is different from Yann ordering fish and Camille ordering chicken.)
In this case, there are 10 ways for Yann to order a dish, and for each of those ways, there are 9 ways for Camille to order a different dish. So, by the multiplication principle, there are 10 times 9 ways for Yann and Camille to order different dishes. Therefore, the number of different combinations of meals is 10 times 9, which is 90. 