1983 AIME Problems/Problem 10
Contents
Problem
The numbers ,
and
have something in common: each is a
-digit number beginning with
that has exactly two identical digits. How many such numbers are there?
Solution
Solution 1
Suppose that the two identical digits are both . Since the thousands digit must be
, only one of the other three digits can be
. This means the possible forms for the number are
![$11xy,\qquad 1x1y,\qquad1xy1$](http://latex.artofproblemsolving.com/6/1/8/618de0dca3122f7ae4c49bd44b021966d9723848.png)
Because the number must have exactly two identical digits, ,
, and
. Hence, there are
numbers of this form.
Now suppose that the two identical digits are not . Reasoning similarly to before, we have the following possibilities:
![$1xxy,\qquad1xyx,\qquad1yxx.$](http://latex.artofproblemsolving.com/2/c/c/2ccebb04bbfa82459be3147685b2846acfc824d8.png)
Again, ,
, and
. There are
numbers of this form.
Thus the answer is .
Solution 2
Consider a sequence of digits instead of a
-digit number. Only looking at the sequences which have one digit repeated twice, we notice that the probability that the sequence starts with 1 is
. This means we can find all possible sequences with one digit repeated twice, and then divide by
.
If we let the three distinct digits of the sequence be and
, with
repeated twice, we can make a table with all possible sequences:
There are possible sequences.
Next, we can see how many ways we can pick ,
, and
. This is
, because there are
digits, from which we need to choose
with regard to order. This means there are
sequences of length
with one digit repeated. We divide by 10 to get
as our answer.
Solution 3 (Complementary Counting)
We'll use complementary counting. We will split up into cases: (1) no number is repeated, (2)
numbers are repeated, and
other numbers are repeated, (3)
numbers are repeated, or (4)
numbers are repeated.
Case 1:
There are choices for the hundreds digit (it cannot be
),
choices for the tens digit (it cannot be
or what was chosen for the hundreds digit), and
for the units digit. This is a total of
numbers.
Case 2:
One of the three numbers must be , and the other two numbers must be the same number, but cannot be
(That will be dealt with in case 4). There are
choices to put the
, and there are
choices (not
) to pick the other number that is repeated, so a total of
numbers.
Case 3:
We will split it into subcases: one where
is repeated
times, and one where another number is repeated
times.
When
is repeated
times, then one of the digits is not
. There are
choices for that number, and
choices for its location,so a total of
numbers.
When a number other than
is repeated
times, then there are
choices for the number, and you don't have any choices on where to put that number.
So in Case 3 there are
numbers
Case 4:
There is only number:
.
There are a total of
-digit numbers that begin with
(from
to
), so by complementary counting you get
numbers.
See Also
1983 AIME (Problems • Answer Key • Resources) | ||
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Followed by Problem 11 | |
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