a) $\dfrac{2}{3} \cdot \dfrac{5}{4}-\dfrac{3}{4} \cdot \dfrac{2}{3}=\dfrac{2}{3} \cdot\left(\dfrac{5}{4}-\dfrac{3}{4}\right)=\dfrac{2}{3} \cdot \dfrac{1}{2}=\dfrac{1}{3}$;
b) $2 \cdot\left(\dfrac{-3}{2}\right)^{2}-\dfrac{7}{2}=2 \cdot \dfrac{9}{4}-\dfrac{7}{2}=\dfrac{9}{2}-\dfrac{7}{2}=1$;
c) $-\dfrac{3}{4} \cdot 5 \dfrac{3}{13}-0,75 \cdot \dfrac{36}{13}=-\dfrac{3}{4} \cdot 5 \dfrac{3}{13}-\dfrac{3}{4} \cdot \dfrac{36}{13}$
$=-\dfrac{3}{4}\left(5 \dfrac{3}{13}+\dfrac{36}{13}\right)$
$=-\dfrac{3}{4} \cdot 8=-6$.

a) $ \dfrac{1}{5}+\dfrac{4}{5}: x=0,75$

$ \dfrac{1}{5}+\dfrac{4}{5}: x=\dfrac{3}{4}$

$\dfrac{4}{5}: x=\dfrac{3}{4}-\dfrac{1}{5}$

$\dfrac{4}{5}: x=\dfrac{11}{20}$

$x=\dfrac{16}{11}$;

b) $x+\dfrac{1}{2}=1-x$

 $2x=1-\dfrac{1}{2}$

 $2x=\dfrac{1}{2}$

 $x=\dfrac{1}{4}$.

Đáp số: 70 khóm hoa