The Q-meter works on the principle of

A reading of 120 is obtained when a standard inductor was connected in the circuit of a Q-meter and the variable capacitor is adjusted to a value of 300pF. A lossless capacitor of unknown value  is then connected in parallel with the variable capacitor and the same reading was obtained when the variable capacitor is readjusted to a value of 200pF. The value of  in pF is