All empty white triangles in figure are to be filled with integers such that for each gray triangle the three numbers in the white neighboring triangles sum to a multiple of $5$. The lower left and the lower right white triangle are already filled with the numbers $12$ and $3$, respectively.
Find all integers that can occur in the uppermost white triangle.

(G. Woeginger, Eindhoven, The Netherlands)
[img]https://cdn.artofproblemsolving.com/attachments/8/a/764732f5debbd58a147e9067e83ba8d31f7ee9.png[/img]
1. **Define Variables:**
   Let \( a, b, c, d \) be the four remaining numbers in the bottom row from left to right, respectively. The given numbers are \( 12 \) and \( 3 \) at the lower left and lower right white triangles, respectively.

2. **Second-Lowest Row:**
   The numbers in the second-lowest row are determined by the sums of the neighboring triangles in the bottom row. These sums must be congruent to multiples of 5. Therefore, we have:
   \[
   3 - a, \quad -a - b, \quad -b - c, \quad -c - d, \quad 2 - d
   \]

3. **Third-Lowest Row:**
   The numbers in the third-lowest row are determined by the sums of the neighboring triangles in the second-lowest row. These sums must also be congruent to multiples of 5. Therefore, we have:
   \[
   2 + 2a + b, \quad a + 2b + c, \quad b + 2c + d, \quad 3 + c + 2d
   \]

4. **Fourth-Lowest Row:**
   The numbers in the fourth-lowest row are determined by the sums of the neighboring triangles in the third-lowest row. These sums must also be congruent to multiples of 5. Therefore, we have:
   \[
   3 + 2a + 2b - c, \quad -a + 2b + 2c - d, \quad 2 - b + 2c + 2d
   \]

5. **Second-Highest Row:**
   The numbers in the second-highest row are determined by the sums of the neighboring triangles in the fourth-lowest row. These sums must also be congruent to multiples of 5. Therefore, we have:
   \[
   2 - a + b - c + d, \quad 3 + a - b + c - d
   \]

6. **Top Triangle:**
   The number in the top triangle is determined by the sum of the neighboring triangles in the second-highest row. This sum must be congruent to a multiple of 5. Therefore, we have:
   \[
   0 \pmod{5}
   \]

   This means the number in the top triangle must be a multiple of 5. Therefore, the possible values for the number in the top triangle are:
   \[
   \boxed{5k} \quad \text{for any} \quad k \in \mathbb{Z}
   \]