Arrange the following fractions in descending order: $\dfrac{7}{30}, \dfrac{13}{15}, \dfrac{9}{10}, \dfrac{3}{5}$

LCM of 30, 15, 10 and 5 = 30.

Write the given fractions as equivalent like fractions.

$\Rightarrow \dfrac{7}{30} = \dfrac{7}{30} \\[1em] \Rightarrow \dfrac{13}{15} = \dfrac{13 \times 2}{15 \times 2} = \dfrac{26}{30} \\[1em] \Rightarrow \dfrac{9}{10} = \dfrac{9 \times 3}{10 \times 3} = \dfrac{27}{30} \\[1em] \Rightarrow \dfrac{3}{5} = \dfrac{3 \times 6}{5 \times 6} = \dfrac{18}{30}$

As 27 > 26 > 18 > 7,

$\dfrac{27}{30} \gt \dfrac{26}{30} \gt \dfrac{18}{30} \gt \dfrac{7}{30} \Rightarrow \dfrac{9}{10} \gt \dfrac{13}{15} \gt \dfrac{3}{5} \gt \dfrac{7}{30}$.

Hence, the given fractions in descending order are $\dfrac{9}{10}, \dfrac{13}{15}, \dfrac{3}{5}, \dfrac{7}{30}$.