Hi, I was doing my homework of Discrete Math course. The subject is binomial, combination and permutation. For 6-7 questions, I have no idea what they mean. For example:

Which rule must be used to find the number of strings of four decimal digits that do not contain the same digit twice?

• the division rule
• the subtraction rule
• the product rule
• the sum rule

How many strings of four decimal digits do not contain the same digit twice?

AND

which rule must be used to find the number of strings of four decimal digits that end with an even digit?

• the division rule
• the sum rule
• the subtraction rule
• the product rule

How many strings of four decimal digits end with an even digit?