Question

explain how you know 21/30 is greater than 2/3

Answer 1

Simple.

to figure out if they are either: equal, greater, or smaller...

First, find a common denominator...

$\frac{21}{30}$

and

$\frac{2}{3} * \frac{10}{10} = \frac{20}{30}$

Now, we can obviously make the assumption that $\frac{21}{30}$ is greater than $\frac{20}{30}$.

Thus, your answer. 

Answer 2

if you do 2 X 10, it's 20.
if you do 3 X 10, it's 30.

so, the fraction would be 20/30, and with the denominators the same, now you can see that 21 is greater than 20.

hopes this helps (: