Arrange the fractions given below in order from greatest to least (Descending order)

$\frac{21}{8},\frac{21}{12},\frac{21}{20},\frac{21}{3}$

Since, the numerator of given fraction are same

Also

$$\begin{gathered} \frac{21}{8}=2.6 \\ \frac{21}{12}=1.7 \\ \frac{21}{20}=1.05 \\ \frac{21}{3}=7 \end{gathered}$$

$\therefore$The descending order of $\frac{21}{8},\frac{21}{12},\frac{21}{20},\frac{21}{3}$is $\frac{21}{3},\frac{21}{8},\frac{21}{12},\frac{21}{20}$