It is a 5 digit number.If we put the numeral 1 at the beginning, we get a number three times smaller than if we put the numeral 1 at the end of the number

Let the 5-digit number be N. If we put 1 at the beginning, we get 100000 + N. If we put 1 at the end, we get 10N + 1. According to the problem: (10N + 1) = 3 × (100000 + N). Solving: 10N + 1 = 300000 + 3N → 7N = 299999 → N = 42857. Verification: 142857 ÷ 428571 = 1/3. The other options don't satisfy this mathematical relationship.