\mathrm{B}_{X}$ and $\mathrm{B}_{\mathrm{Y}}$ are the magnetic fields at the centre of two coils $\mathrm{X}$ and $\mathrm{Y}$ respectively each carrying equal current. If coil $X$ has 200 turns and $20 \mathrm{~cm}$ radius and coil $Y$ has 400 turns and $20 \mathrm{~cm}$ radius, the ratio of $B_{X}$ and $B_{Y}$ is :