Difference between revisions of "1983 AIME Problems/Problem 10"
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Thus, the desired answer is <math>216+216=432</math>. | Thus, the desired answer is <math>216+216=432</math>. | ||
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* [[AIME Problems and Solutions]] | * [[AIME Problems and Solutions]] | ||
Revision as of 07:01, 16 April 2012
Problem
The numbers 
, 
, and 
have something in common. Each is a four-digit number beginning with 
that has exactly two identical digits. How many such numbers are there?
Solution
Suppose the two identical digits are both one. Since the thousands digits must be one, the other one can be in only one of three digits,
Because the number must have exactly two identical digits, 
, 
, and 
. Hence, there are 
numbers of this form.
Suppose the two identical digits are not one. Therefore, consider the following possibilities,
Again, , 
, and 
. There are numbers of this form as well.
Thus, the desired answer is 
.
See Also
| 1983 AIME (Problems • Answer Key • Resources) | ||
| Preceded by Problem 9 |
Followed by Problem 11 | |
| 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
| All AIME Problems and Solutions | ||
