Arrange the following in descending order: (i) $\frac{2}{9}, \frac{2}{3}, \frac{8}{21}$ (ii) $\frac{1}{5}, \frac{3}{7}, \frac{7}{10}$

(i) To arrange in descending order, we need to convert the unlike fractions into like fractions: 

The LCM of $9, 3, 21$ is $63$

$\frac{2}{9}\times \frac{7}{7}=\frac{14}{63}; \frac{2}{3}\times \frac{21}{21}=\frac{42}{63}; \frac{8}{21}\times \frac{3}{3}=\frac{24}{63}$

$\begin{array}{rcl}\frac{42}{63}& >& \frac{24}{63}>\frac{14}{63}\\ \frac{2}{3}& >& \frac{8}{21}>\frac{2}{9}\end{array}$

Hence, the fractions in descending order are: $\frac{2}{3}, \frac{8}{21}, \frac{2}{9}$

(ii) To arrange in descending order, we need to convert the unlike fractions into like fractions: 

The LCM of $5, 7, 10$ is $70$

$\frac{1}{5}\times \frac{14}{14}=\frac{14}{70}; \frac{3}{7}\times \frac{10}{10}=\frac{30}{70}; \frac{7}{10}\times \frac{7}{7}=\frac{49}{70}$

$\begin{array}{rcl}\frac{49}{70}& >& \frac{30}{70}>\frac{14}{70}\\ \frac{7}{10}& >& \frac{3}{7}>\frac{1}{5}\end{array}$

Hence, the fractions in descending order are: $\frac{7}{10}, \frac{3}{7}, \frac{1}{5}$