§145

To say that the student (B) has understood how to write the series the student would have had to have repeatedly done it successfully themselves. A criterion of understanding is the person correctly writing out the series themselves.
You can then point out to them that the numbers 0-9 recur in the numbers from 10 upwards - in the units and in the tens.

When do we say they've mastered the system? - There is no clear limit up to which they must be able to complete the series in order for us to have a right to say they've understood/mastered it.