Two concentric circular coils, $C_1$ and $C_2$ are placed in the XY plane. $\mathrm{C}_1$ has 500 turns, and a radius of $1 \mathrm{~cm} . \mathrm{C}_2$ has 200 turns and radius of $20 \mathrm{~cm} . \mathrm{C}_2$ carries a time dependent current $\mathrm{I}(\mathrm{t})=\left(5 \mathrm{t}^2-2 \mathrm{t}+3\right)$ A where t is in s . The emf induced in $\mathrm{C}_1$ (in mV ), at the instant $t=1 \mathrm{~s}$ is $\frac{4}{x}$. The value of $x$ is