Reciprocal of 21

Theorem
The decimal expansion of the reciprocal of $21$ has period $6$:
 * $\dfrac 1 {21} = 0 \cdotp \dot 04761 \, \dot 9$

Proof
Performing the calculation using long division:

0.04761901... -- 21)1.00000000000     84     --     160     147     ---      130      126      ---        40        21       ---        190        189        ---          100           84          ---          ...