Let $x_1,x_2,y_1,y_2$ be real numbers satisfying the equations $x^2_1+5x^2_2=10$, $x_2y_1-x_1y_2=5$, and $x_1y_1+5x_2y_2=\sqrt{105}$. Find the value of $y_1^2+5y_2^2$
1. Given the equations:
   \[
   x_1^2 + 5x_2^2 = 10
   \]
   \[
   x_2y_1 - x_1y_2 = 5
   \]
   \[
   x_1y_1 + 5x_2y_2 = \sqrt{105}
   \]

2. We introduce complex numbers to simplify the problem. Let:
   \[
   x = x_1 + i\sqrt{5}x_2
   \]
   \[
   y = y_1 - i\sqrt{5}y_2
   \]

3. The first equation gives the magnitude of \( x \):
   \[
   |x|^2 = x_1^2 + (\sqrt{5}x_2)^2 = x_1^2 + 5x_2^2 = 10
   \]
   Therefore:
   \[
   |x| = \sqrt{10}
   \]

4. Consider the product \( xy \):
   \[
   xy = (x_1 + i\sqrt{5}x_2)(y_1 - i\sqrt{5}y_2)
   \]
   Expanding this, we get:
   \[
   xy = x_1y_1 - i\sqrt{5}x_1y_2 + i\sqrt{5}x_2y_1 - 5x_2y_2
   \]
   Simplifying, we have:
   \[
   xy = (x_1y_1 + 5x_2y_2) + i\sqrt{5}(x_2y_1 - x_1y_2)
   \]

5. From the given equations, we know:
   \[
   x_2y_1 - x_1y_2 = 5
   \]
   \[
   x_1y_1 + 5x_2y_2 = \sqrt{105}
   \]

6. Substituting these into the expression for \( xy \):
   \[
   xy = \sqrt{105} + i\sqrt{5} \cdot 5
   \]
   \[
   xy = \sqrt{105} + 5i\sqrt{5}
   \]

7. The magnitude of \( xy \) is:
   \[
   |xy| = \sqrt{(\sqrt{105})^2 + (5\sqrt{5})^2}
   \]
   \[
   |xy| = \sqrt{105 + 125}
   \]
   \[
   |xy| = \sqrt{230}
   \]

8. Since \( |x| = \sqrt{10} \), we have:
   \[
   |xy| = |x| \cdot |y|
   \]
   \[
   \sqrt{230} = \sqrt{10} \cdot |y|
   \]
   Solving for \( |y| \):
   \[
   |y| = \frac{\sqrt{230}}{\sqrt{10}} = \sqrt{23}
   \]

9. The magnitude \( |y| \) is given by:
   \[
   |y|^2 = y_1^2 + (\sqrt{5}y_2)^2 = y_1^2 + 5y_2^2
   \]
   Therefore:
   \[
   y_1^2 + 5y_2^2 = 23
   \]

The final answer is \( \boxed{23} \)