How could you explain that 21/30 is greater than 2/3?

Well its greater than 2/3, because 2/3 = 20/30, so 21/30 is greater.

To begin with, 2/3 can also be written as 20/30

Now we see that 21 is greater than 20.

So, in this one way we can say that 21/30 is slightly greater than 2/3.

Still didnt understand?

Lets go deep down, Maybe converting the fraction to a decimal will be good.

So when we convert the fraction 21/30, it gives us the decimal value:0.7

The same thing with 2/3, it gives us a non-ending decimal value:0.6667

However, dont get confused by the fact that 0.66667 is so close to 0.7

Although it is close enough, 0.7 is still higher.

In closure, 21/30 is greater than 2/3