Question: how many possible combinations of 5 numbers without repeating?
Answer: Thus you have made 5 × 4 × 3 × 2 1 = 120 choices and there are 120 possible 5 digit numbers made from 1, 2, 3, 4 and 5 if you don't allow any digit to be repeated. Now consider the possibilities with 13 as the first two digits.