A series LCR circuit with $R = {{250} \over {11}}\,\Omega $ and ${X_L} = {{70} \over {11}}\,\Omega $ is connected across a 220 V, 50 Hz supply. The value of capacitance needed to maximize the average power of the circuit will be _________ $\mu$F. (Take : $\pi = {{22} \over 7}$)