Which of the following is a prime number?

How many different ways can I make a 4-digit passcode for my phone using the numbers 0-9?

I want a pizza with 3 toppings, and I have 10 topping choices. How many different pizzas can I make if I don't want two or more of the same topping?

I have 5 ice cream flavors (chocolate, strawberry, vanilla, banana, mango). How many cones can I make with 3 cones?

I have 3 salad ingredients (lettuce, tomatoes, carrots). How many different salads can I make with 2 different ingredients?