2003 AMC 10B Problems/Problem 25
Problem
How many distinct four-digit numbers are divisible by and have
as their last two digits?
Solution
Solution 1
To test if a number is divisible by three, you add up the digits of the number. If the sum is divisible by three, then the original number is a multiple of three. If the sum is too large, you can repeat the process until you can tell whether it is a multiple of three. This is the basis for the solution. Since the last two digits are , the sum of the digits is
(therefore it is not divisible by three). However certain numbers can be added to make the sum of the digits a multiple of three.
,
, and so on.
However since the largest four-digit number ending with is
, the maximum sum is
.
Using that process we can fairly quickly compile a list of the sum of the first two digits of the number.
Now we find all the two-digit numbers that have any of the sums shown above. We can do this by listing all the two digit numbers in separate cases.
And finally, we add the number of elements in each set.
Solution 2
A number divisible by has all its digits add to a multiple of
The last two digits are
and
and add up to
Therefore the first two digits must add up to
digits (including
) are
are
and
are
The following combinations are equivalent to
:
Let the first term in each combination represent the thousands digit and the second term represent the hundreds digit. We can use this to find the total amount of four-digit numbers.
Solution 3
We have the following: and
. Then
for some integer
and
. Taking mod 3 gives:
so
for some integer
. But
so
. Bounding this gives us:
so
. Dividing by
gives
so
. This gives
\\
Solution by forthegreatergood
See also
2003 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 24 |
Followed by Last problem | |
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