Question

Assume that you select digits from the set {1,2,4,9}{1,2,4,9} to form a number between 10 and 900. (1) how many such numbers can be formed if digits are not repeated? 12 equation editorequation editor (2) how many such numbers can be formed if digits can be repeated?

Answer 1

1)

2-digit number or 3-digit number

4 options x 3 remaining options + 4 options x 3 options x 2 options

= 12 + 24

= 36

Answer: 36 different numbers

2)

2-digit number or 3-digit number

4 options x 4 options + 4 options x 4 options x 4 options

= 16 + 64

= 80

Answer: 80 different numbers