a) $$5 \cdot 2\frac{1}{10} = 5 \cdot (2 + \frac{1}{10}) = 5 \cdot 2 + 5 \cdot \frac{1}{10} = 10 + \frac{5}{10} = 10 + \frac{1}{2} = 10\frac{1}{2}$$
Ответ: $$10\frac{1}{2}$$