По теореме синусов:
$$\frac{a}{sin \alpha} = 2R$$ $$R = \frac{a}{2sin \alpha}$$ $$R = \frac{24}{2 \cdot \frac{4}{5}}$$ $$R = \frac{24}{1} \cdot \frac{5}{8}$$ $$R = \frac{3 \cdot 5}{1}$$ $$R = 15$$
Ответ: R = 15