Millionth Digit of the Counting Numbers
Date: 02/26/2001 at 19:58:24 From: Keith Bukovich Subject: Counting numbers 1,000,000th digit My friend and I have a disagreement on the answer to this question that popped up in class: "A number is formed by writing the counting numbers in order: 123456789101112131415... What is the one millionth digit in this number?" She says it is 1 and I thought it might be 0. I looked for a pattern. 1- 2- 3- 4- 5- 6- 7- 8- 9-10 11-12-13-14-15-16-17-18-19-20 21-22-23-24-25-26-27-28-29-30 Every tenth number ends in zero and one million is divisible by ten. Then it occurred to me that she might be right, since I was considering values, not digits. Still, I can't seem to find the pattern for 1 to be the answer. Please let me know who is right. Also, if 1 is the answer, what is the pattern? Thanks for any and all consideration. Regards, Keith
Date: 02/26/2001 at 20:31:12 From: Doctor Schwa Subject: Re: Counting numbers 1,000,000th digit > 1- 2- 3- 4- 5- 6- 7- 8- 9-10 > 11-12-13-14-15-16-17-18-19-20 > 21-22-23-24-25-26-27-28-29-30 > >Every tenth number ends in zero and one million is divisible by ten. >Then it occurred to me that she might be right, since I was >considering values not digits. That's exactly right. In the first line of your example, it's true that the tenth number ends in 0, but the ELEVENTH digit of the list is the 0. Then on the next line, you have twenty more digits, so the 31st digit is again zero, and so on. >Still I can't seem to find the pattern for 1 to be the answer. Please >let me know who is right. Also, if 1 is the answer, what is the >pattern? The strategy to solve this problem is to count the digits carefully, like this (I'll find the ten thousandth digit in the list as an example): 9 digits used up on the one-digit numbers. 2*90 = 180 digits used up on the two-digit numbers (so the 189th digit in the list is the last 9 of 99, and the 190th digit is the first 1 of 100). 3 * 900 = 2700 digits used up on the three-digit numbers. So the 2889th digit is the last 9 of 999, and the 2890th digit is the first 1 of 1000. Now, the 2894th digit is the first 1 of 1001, and so on; the (2890 + 4n) digit is the first digit of (1000 + n), so the 9998th digit is when n = 1777. This makes the 10000th digit the third digit of 2777, which is a 7. Similarly the 10001st digit is a 7, and the 10002nd digit is a 2 (the first digit of 2778). Now keep going with that same kind of reasoning and see if you can find the millionth digit in the list. - Doctor Schwa, The Math Forum http://mathforum.org/dr.math/
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