With a 5-digit number, you have 5 choices for the first digit. There will be 4 digits left so you have 4 choices for the second digit. After you have chosen the first 2 digits there will be 3 remaining so you will have 3 choices for the third digit. Hence there are 2 choices for the fourth digit and only 1 choice for the fifth digit. Thus you have made 5 × 4 × 3 × 2 1 = 120 choices and there are 120 possible 5 digit numbers made from 1, 2, 3, 2 and 1
