15. Calculate $$2\sqrt{13} \cdot 5\sqrt{2} \cdot \sqrt{26}$$.

Let's break this down step-by-step:

1. We need to calculate the product $$2\sqrt{13} \cdot 5\sqrt{2} \cdot \sqrt{26}$$.
2. First, multiply the coefficients (the numbers outside the square roots): $$2 \times 5 = 10$$.
3. Next, multiply the numbers inside the square roots: $$\sqrt{13} \times \sqrt{2} \times \sqrt{26}$$.
4. Combine these under a single square root: $$\sqrt{13 \times 2 \times 26}$$.
5. Calculate the product inside the square root: $$13 \times 2 = 26$$.
6. So, we have $$\sqrt{26 \times 26}$$.
7. This is $$\sqrt{26^2}$$, which equals 26.
8. Now, combine the coefficient and the result from the square root: $$10 \times 26$$.
9. $$10 \times 26 = 260$$.

Ответ: 260