12. 4 a^1/15 ⋅ √a.

$$a^{\frac{1}{15}} \cdot \sqrt{a} = a^{\frac{1}{15}} \cdot a^{\frac{1}{2}} = a^{\frac{1}{15} + \frac{1}{2}} = a^{\frac{2 + 15}{30}} = a^{\frac{17}{30}}$$

Ответ: $$a^{\frac{17}{30}}$$