If a lock has 4 wheels each labelled with digits 0-9, how many different combinations can be formed using 4 digits without repetition?

If a student has 4 different books and wants to choose 2 to read, how many ways can he choose them?

A shirt can be chosen in 3 different ways, and a pant can be chosen in 5 different ways. How many different outfits can be made?

How many different 3-digit numbers can be formed using the digits 1, 2, 3, and 4 with no repetition?

If an event occurs in 5 different ways and is followed by another event that can occur in 3 different ways, what is the total number of occurrences?