How many different 4-letter codes can be formed from the letters A, B, C, and D if letters can be repeated?

In how many ways can 5 different books be arranged on a shelf?

A school club has 10 members. In how many ways can a president, vice-president, and treasurer be elected from these members?

A lock combination consists of 3 distinct digits chosen from 0 to 9. How many different lock combinations are possible if no digit may be repeated?

How many different 4-digit PIN numbers can be created using the digits 1 through 4 if each digit can only be used once?