In a parallel circuit, the total circuit current equals the sum of the individual currents.

If the total current of a parallel circuit containing a lamp and radio is $\displaystyle \frac{{2}}{{7}}$ amps, and the lamp's current is $\displaystyle \frac{{1}}{{21}}$ amps, find the current of the radio.

Give your answer as a fraction, reduced to lowest terms. No need to enter "amps".