Two long parallel transmission lines $40.0 cm$apart carry $25.0 A$ and $75.0 A$currents. Find the locations where the ent magnetic field of the two wires is zero if these currents are in same direction?

If currents are in the same direction, then along and below the wires magnetic fields are in the same direction. Hence, they can't produce zero magnetic fields. In between the wires, let $B=0$ at a distance $r cm$ from the wire carrying $75 A$ current. Then, $\frac{{\mu }_0}{2\mathrm{\pi }}\left(\frac{75}{r}\right)=\left(\frac{{\mu }_0}{2\mathrm{\pi }}\right)\left(\frac{25}{40-\mathrm{\pi }}\right)$

Solving, we get $r=30 cm$