For a given series LCR circuit it is found that maximum current is drawn when value of variable capacitance is $$25 \text{ nF}$$. If resistance of $$200 \Omega$$ and $$100 \text{ mH}$$ inductor is being used in the given circuit. The frequency of ac source is ___________ $$\times 10^3 \text{ Hz}$$. (given $$\pi^2 = 10$$)