a)

• $A = -1,6 : (1 + \frac{2}{5}) = \frac{-16}{10} : (\frac{5}{5} + \frac{2}{5})$

$ = \frac{-8}{5} : \frac{7}{5} = \frac{-8}{5} \times \frac{5}{7} = \frac{-8}{7}$;

• $B = 1,4\times \frac{15}{49} - (\frac{4}{5} - \frac{2}{3}) : 2\frac{1}{5} = \frac{14}{10} \times \frac{15}{49} - (\frac{12}{15} + \frac{10}{15}) : \frac{11}{5}$

$ = \frac{7}{5}\times \frac{15}{49} - \frac{22}{15}\times \frac{5}{11} = \frac{3}{7} - \frac{2}{3} = \frac{9}{21} - \frac{14}{21} = \frac{5}{21}$;

b)

• $(2,8x - 32) : \frac{2}{7} = -91$;

$\Leftrightarrow 2,8x - 32 = -91\times \frac{2}{7}$

$\Leftrightarrow \frac{28x}{10} - 32 = \frac{-182}{7}$

$\Leftrightarrow \frac{14x}{5} = \frac{-182}{7} + 32$

$\Leftrightarrow \frac{14x}{5} = \frac{42}{7}$

$\Leftrightarrow \frac{x}{5} = \frac{3}{7}$

$\Leftrightarrow x = \frac{3}{7}\times 5$

$\Leftrightarrow x = \frac{15}{7}$

• $(3,5 - 2x)\times 1\frac{4}{7} = \frac{11}{14}$;

$\Leftrightarrow 3,5x - 2x = \times \frac{11}{14} : 1\frac{4}{7}$

$\Leftrightarrow \frac{7x}{2} - 2x = \times \frac{11}{14} : \frac{11}{7}$

$\Leftrightarrow \frac{7x}{2} - \frac{4x}{2}  = \times \frac{11}{14} \times \frac{7}{11}$

$\Leftrightarrow \frac{3x}{2} = \frac{1}{2}$

$\Leftrightarrow 3x = 1$

$\Leftrightarrow x = \frac{1}{3}$

c) Đổi: 356,5 (m vải) = $\frac{713}{2}$ (m).

Gọi x là số mét vải hoa, (mét vải);

$\Rightarrow (\frac{713}{2} - x)$ là số mét vải trắng.

Theo đề bài: $x = 78,25\%\times (\frac{713}{2} - x)$

$\Leftrightarrow x = \frac{78,25}{100}\times (\frac{713}{2} - x)$

$\Leftrightarrow x = \frac{\frac{313}{4}}{100}\times (\frac{713}{2} - x)$

$\Leftrightarrow x = \frac{313}{400}\times \frac{713}{2} - \frac{313x}{400}$

$\Leftrightarrow \frac{713x}{400} = \frac{313}{400}\times \frac{713}{2}$

$\Leftrightarrow x = \frac{313}{2}$

Vậy số mét vải hoa là: $\frac{313}{2}$ (mét vải); Số mét vải trắng là: $\frac{713}{2} - \frac{313}{2} = \frac{400}{2} = 200$ (mét vải).