2009 AMC 12A Problems/Problem 12
Problem
How many positive integers less than are
times the sum of their digits?
Solution
Solution 1
The sum of the digits is at most . Therefore the number is at most
. Out of the numbers
to
the one with the largest sum of digits is
, and the sum is
. Hence the sum of digits will be at most
.
Also, each number with this property is divisible by , therefore it is divisible by
, and thus also its sum of digits is divisible by
.
We only have six possibilities left for the sum of the digits: ,
,
,
,
, and
. These lead to the integers
,
,
,
,
, and
. But for
the sum of digits is
, which is not
, therefore
is not a solution. Similarly we can throw away
,
,
, and
, and we are left with just
solution: the number
.
Solution 2
We can write each integer between and
inclusive as
where
and
.
The sum of digits of this number is
, hence we get the equation
. This simplifies to
. Clearly for
there are no solutions, hence
and we get the equation
. This obviously has only one valid solution
, hence the only solution is the number
.
Solution 3
The sum of the digits is at most . Therefore the number is at most
. Since the number is
times the sum of its digits, it must be divisible by
, therefore also by
, therefore the sum of its digits must be divisible by
. Next we can conclude that the number must be divisible by
, not just by
. Since the number is divisible by
, it is also divisible by
, therefore the sum of its digits is divisible by
, therefore the number is divisible by
, which leaves us with
,
and
. Only
is
times its digits, hence the answer is
.
See Also
2009 AMC 12A (Problems • Answer Key • Resources) | |
Preceded by Problem 11 |
Followed by Problem 13 |
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 | |
All AMC 12 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.