2007 AMC 12A Problems/Problem 16
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Problems
How many three-digit numbers are composed of three distinct digits such that one digit is the average of the other two?
Solution
We can find the number of increasing arithmetic sequences of length 3 possible from 0 to 9, and then find all the possible permutations of these sequences.
| Common difference | Sequences possible | Number of sequences |
| 1 |  | 8 |
| 2 |  | 6 |
| 3 |  | 4 |
| 4 |  | 2 |
This gives us a total of 
sequences. There are 
to permute these, for a total of 
.
However, we note that the conditions of the problem require two digit numbers, and hence our numbers cannot start with zero. There are 
numbers which start with zero, so our answer is 
.
See also
| 2007 AMC 12A (Problems) | ||
| Preceded by Problem 15 | Followed by Problem 17 | |
| 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 | ||
