13. Calculate $$\sqrt{18 \cdot 72 \cdot \sqrt{16}}$$.

Let's solve this step-by-step:

1. The expression is $$\sqrt{18 \cdot 72 \cdot \sqrt{16}}$$.
2. First, calculate the innermost square root: $$\sqrt{16} = 4$$.
3. Now, substitute this back into the expression: $$\sqrt{18 \cdot 72 \cdot 4}$$.
4. Multiply the numbers inside the square root: $$18 \times 72 \times 4$$.
5. Let's do $$72 \times 4 = 288$$.
6. Now, $$18 \times 288$$.
7. $$18 \times 288 = 5184$$.
8. So, we need to find $$\sqrt{5184}$$.
9. Let's try to estimate. We know $$70^2 = 4900$$ and $$80^2 = 6400$$. So the answer is between 70 and 80.
10. The last digit of 5184 is 4, so the square root must end in 2 or 8 (since $$2^2=4$$ and $$8^2=64$$).
11. Let's try 72: $$72 \times 72$$.
12. $$72 \times 72 = (70+2)(70+2) = 70^2 + 2(70)(2) + 2^2 = 4900 + 280 + 4 = 5184$$.
13. So, $$\sqrt{5184} = 72$$.

Ответ: 72