What should be added to $\dfrac{2}{5}$ to get -1 ?

What should be subtracted from $\dfrac{-3}{4}$ to get $\dfrac{-5}{6}$

Multiply :

(i) $\dfrac{2}{3}$ by $\dfrac{4}{5}$

(ii) $\dfrac{7}{6}$ by $\dfrac{9}{2}$

(iii) $\dfrac{5}{6}$ by 30

(iv) $\dfrac{-3}{4}$ by $\dfrac{8}{7}$

(v) $\dfrac{-16}{9}$ by $\dfrac{12}{-5}$

(vi) $\dfrac{35}{-8}$ by $\dfrac{12}{-5}$

(vii) $\dfrac{-3}{10}$ by $\dfrac{-40}{9}$

(viii) $\dfrac{-32}{5}$ by $\dfrac{15}{-16}$

(ix) $\dfrac{-8}{15}$ by $\dfrac{-25}{32}$

Simplify :

(i) $\dfrac{7}{15} \times \dfrac{5}{6}$

(ii) $\dfrac{-5}{24} \times \dfrac{6}{25}$

(iii) $\dfrac{7}{-18} \times \dfrac{-9}{14}$

(iv) $\dfrac{-9}{5} \times \dfrac{-10}{3}$

(v) $-28 \times \dfrac{-8}{7}$

(vi) $\dfrac{8}{-21} \times \dfrac{-14}{3}$