в) $$ \frac{3}{7}a + \frac{2}{5} = 1 $$
$$ \frac{3}{7}a = 1 - \frac{2}{5} $$
$$ \frac{3}{7}a = \frac{5}{5} - \frac{2}{5} $$
$$ \frac{3}{7}a = \frac{3}{5} $$
$$ a = \frac{3}{5} : \frac{3}{7} $$
$$ a = \frac{3}{5} \cdot \frac{7}{3} $$
$$ a = \frac{3 \cdot 7}{5 \cdot 3} $$
$$ a = \frac{7}{5} $$
$$ a = 1\frac{2}{5} $$
Ответ: 1$$\frac{2}{5}$$