Associated Topics || Dr. Math Home || Search Dr. Math

### Lucky Number Sequences

```
Date: 05/11/98 at 05:54:10
From: Matt Dellit
Subject: Lucky Number

A lucky number is one for which the sum of its digits is divisible by
7. For example, 7, 25 and 849 are lucky numbers. The smallest pair of
lucky numbers is 7 and 16; they differ by 9.

The problems:

a) Find eight consecutive numbers two of which are lucky.

b) Find 12 consecutive numbers none of which is lucky.

c) Show that 13 consecutive numbers always contain at least one
lucky number.

d) "Hey," says Chris, "I've found the largest pair of lucky numbers
that differ by 9!" Paris replies, "Chris, you're wrong!" Explain
how Paris knows Chris is wrong.

Thanks for this.

Matt Dellit
```

```
Date: 05/12/98 at 13:36:56
From: Doctor Dennis
Subject: Re: Lucky Number

(160 and 167 are lucky). Basically, my idea was to just try bigger
numbers until finding an answer. Also, realizing that numbers with
digit sum 7 must be at least 9 apart.

For problem b, I found 994-1005, using basically more guesswork. If
you play with the numbers for a while, it becomes clear that most of
the time there is a lucky number every 9 numbers, so to get every 12
we need to find a "strange" area in terms of the sum of digits. One
such strange area is the 999-1000 line, where the digit sum changes
drastically. Probably there are many other similar areas, but I
couldn't think of any with 2 or fewer digits.

In problem c, each time we change the ones digit only, we increase the
sum, and hence the remainder of division by 7, by 1. The remainder of
division by 7 can only be 1 through 6 (0 means we have a lucky number,
which we don't want). Also, within 13 consecutive numbers we can only
change digits other than the ones digit once (meaning we carried to
the next digit), but when we change other digits we can reset the
remainder of division by 7. For example, using the numbers from
problem b:

number    remainder when we divide by 7
----------------------------------------
994                  1
995                  2
996                  3
997                  4
998                  5
999                  6
1000                  1
1001                  2
1002                  3
1003                  4
1004                  5
1005                  6

And we find 12 numbers in a row that are not lucky numbers. Thinking
about all this, can you see why 13 numbers must contain one lucky
number? The basic idea here is to try to construct as long a sequence
as possible and show that it can only have 12 numbers without a
lucky number.

For problem d, consider the pair 151 and 160. These differ by 9. So do
1051 and 1060. As do 10051 and 10060. And 10000000000000051 and
10000000000000060. This should set off some ideas about why Chris
is wrong.

Let me know if you have any more questions.

-Doctor Dennis,  The Math Forum
Check out our web site! http://mathforum.org/dr.math/
```
Associated Topics:
High School Number Theory

Search the Dr. Math Library:

 Find items containing (put spaces between keywords):   Click only once for faster results: [ Choose "whole words" when searching for a word like age.] all keywords, in any order at least one, that exact phrase parts of words whole words

Submit your own question to Dr. Math
Math Forum Home || Math Library || Quick Reference || Math Forum Search