Since there are 5 appetizers and 10 main courses, that already gives us 50 options. For each appetizer, you can pair it with ten different courses. That means for each appetizer, 10 options are possible. 10*5=50, so if there were only appetizers and courses, there'd be 50 choices. But there are also desserts. That gives us 2 times the possibilities (since for each previous possibility, you have two options for dessert). 2*50=100. There are 100 options.
