12. Стоимость Z равна неизвестному числу, выраженному в $$3\sqrt{5}$$ и $$5\sqrt{6}$$.

The problem states that the cost of Z is an unknown number, expressed in $$3\sqrt{5}$$ and $$5\sqrt{6}$$. This phrasing is ambiguous and does not provide a clear mathematical operation to combine these values. It could mean several things:

• Sum: $$Z = 3\sqrt{5} + 5\sqrt{6}$$
• Difference: $$Z = |3\sqrt{5} - 5\sqrt{6}|$$ or $$Z = 3\sqrt{5} - 5\sqrt{6}$$ or $$Z = 5\sqrt{6} - 3\sqrt{5}$$
• Product: $$Z = (3\sqrt{5}) \times (5\sqrt{6})$$
• Division: $$Z = \frac{3\sqrt{5}}{5\sqrt{6}}$$ or $$Z = \frac{5\sqrt{6}}{3\sqrt{5}}$$

Without further clarification on how these two values are combined to represent the cost of Z, it's impossible to provide a definitive numerical answer.

Ответ: Недостаточно информации для решения.