Find all ordered pairs $(a, b)$ of positive integers such that $a^2 + b^2 + 25 = 15ab$ and $a^2 + ab + b^2$ is prime.
1. We start with the given equation:
   \[
   a^2 + b^2 + 25 = 15ab
   \]
   Rearrange it to:
   \[
   a^2 + b^2 - 15ab + 25 = 0
   \]

2. We also need to ensure that \(a^2 + ab + b^2\) is prime.

3. Let's solve the quadratic equation \(a^2 + b^2 - 15ab + 25 = 0\) for \(a\) and \(b\). We can rewrite it as:
   \[
   a^2 + b^2 + 25 = 15ab
   \]

4. To find the possible values of \(a\) and \(b\), we can use the factorization approach. Consider the expression:
   \[
   17(a^2 + b^2 + ab) = (4a + 4b + 5)(4a + 4b - 5) + a^2 + b^2 + 25 - 15ab
   \]
   Simplifying, we get:
   \[
   17(a^2 + b^2 + ab) = (4a + 4b + 5)(4a + 4b - 5)
   \]

5. Since \(17(a^2 + b^2 + ab)\) must be a product of two factors, we consider the possible values for these factors. Let:
   \[
   (4a + 4b + 5)(4a + 4b - 5) = 17p
   \]
   where \(p\) is a prime number.

6. We analyze the possible values for \(4a + 4b + 5\) and \(4a + 4b - 5\):
   - If \(4a + 4b - 5 = 1\), then:
     \[
     4a + 4b = 6 \implies a + b = \frac{6}{4} = \frac{3}{2}
     \]
     which is impossible since \(a\) and \(b\) are positive integers.

   - If \(p > 17\), then:
     \[
     4a + 4b - 5 = 17 \implies 4a + 4b = 22 \implies a + b = \frac{22}{4} = \frac{11}{2}
     \]
     which is also impossible.

   - If \(p < 17\), then:
     \[
     4a + 4b + 5 = 17 \implies 4a + 4b = 12 \implies a + b = 3
     \]

7. Given \(a + b = 3\), we test the possible pairs \((a, b)\):
   - If \(a = 1\) and \(b = 2\), then:
     \[
     a^2 + b^2 + 25 = 1^2 + 2^2 + 25 = 1 + 4 + 25 = 30
     \]
     \[
     15ab = 15 \cdot 1 \cdot 2 = 30
     \]
     Thus, \((1, 2)\) is a solution.

   - If \(a = 2\) and \(b = 1\), then:
     \[
     a^2 + b^2 + 25 = 2^2 + 1^2 + 25 = 4 + 1 + 25 = 30
     \]
     \[
     15ab = 15 \cdot 2 \cdot 1 = 30
     \]
     Thus, \((2, 1)\) is a solution.

8. Finally, we need to check if \(a^2 + ab + b^2\) is prime for these pairs:
   - For \((1, 2)\):
     \[
     a^2 + ab + b^2 = 1^2 + 1 \cdot 2 + 2^2 = 1 + 2 + 4 = 7
     \]
     which is prime.

   - For \((2, 1)\):
     \[
     a^2 + ab + b^2 = 2^2 + 2 \cdot 1 + 1^2 = 4 + 2 + 1 = 7
     \]
     which is also prime.

Thus, the ordered pairs \((a, b)\) that satisfy both conditions are \((1, 2)\) and \((2, 1)\).

The final answer is \(\boxed{(a, b) \in \{(1, 2), (2, 1)\}}\)