Two parallel long wires A and B carry currents i_1 and i_2( < i_1)...

Two parallel long wires $A$ and $B$ carry currents $i_1$ and $i_2\left( < i_1\right)$. When $i_1$ and $i_2$ are in the same direction, the magnetic field at a point mid way between the wires is $10 \mu \mathrm{T}$. If $i_2$ is reverse the field becomes $30 \mu \mathrm{T}$. The ratio $i_1 / i_2$ is