2005 AMC 12A Problems/Problem 11
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Problem
How many three-digit numbers satisfy the property that the middle digit is the average of the first and the last digits?
Contents |
Solution
Solution 1
Let the digits be 
so that 
. In order for this to be an integer, 
and 
have to have the same parity. There are 
possibilities for , and 
for . 
depends on the value of both and and is unique for each 
. Thus our answer is 
.
Solution 2
Thus, the three digits form an arithmetic sequence.
- If the numbers are all the same, then there are possible three-digit numbers.
- If the numbers are different, then we count the number of strictly increasing arithmetic sequences between
and 
and multiply by 2 for the decreasing ones:
| Common difference | Sequences possible | Number of sequences |
| 1 |  | 8 |
| 2 |  | 6 |
| 3 |  | 4 |
| 4 |  | 2 |
This gives us 
. However, the question asks for three-digit numbers, so we have to ignore the four sequences starting with . Thus our answer is 
.
See also
| 2005 AMC 12A (Problems • Resources) | ||
| Preceded by Problem 10 | Followed by Problem 12 | |
| 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 | ||
