Product rule for counting ~ Number

Question 1

How many possible combinations of three digit numbers can you make from these 3 cards:

4, 5 and 6?

Answer 1

n = total number of cards

Possible combos = n x (n-1)
= 3 x 2
= 6

Question 2

How many 4 digit pin codes can be made out of the numbers 0-9?

Answer 2

0, 1, 2, 3, 4, 5, 6, 7, 8, 9 = 10 possible numbers for each digit of the code.
Therefore, the answer = 10 x 10 x 10 x 10 = 10,000

Question 3

How many 4 digit pin codes can be made out of the numbers 0-9 when NO DIGIT CAN BE REPEATED?

Answer 3

0, 1, 2, 3, 4, 5, 6, 7, 8, 9 = 10 possible numbers for the first digit.

10 x 9 x 8 x 7 = 5040