$$k - \frac{2}{5}k = 4,5$$
$$\frac{5}{5}k - \frac{2}{5}k = 4,5$$
$$\frac{3}{5}k = 4,5$$
$$k = 4,5 : \frac{3}{5}$$
$$k = 4,5 \cdot \frac{5}{3}$$
$$k = \frac{45}{10} \cdot \frac{5}{3}$$
$$k = \frac{9}{2} \cdot \frac{1}{3} \cdot 5 = \frac{3}{2} \cdot 5 = \frac{15}{2} = 7,5$$
Ответ: 7,5