A circular coil of radius $r$ carries a current $I$. The magnetic field at its centre is $B$. At what distance from the centre, on the axis of the coil the magnetic field will be $\frac{B}{8}$?

Two very thin metallic wires placed along $X$- and $Y$-axis carry equal currents as shown here. $AB$ and $CD$ are lines at $45°$ with the axes with origin of axes at $O$. The magnetic field will be zero on the line,