At a certain place the angle of dip is 30 ° and the horizontal...

Angle of dip, $\delta = 30 °$

Angle of dip or magnetic dip is the angle the net earth's magnetic field and it's horizontal component.

${B}_{H} = Bcos\delta$

$B = \dfrac{{B}_{H}}{cos\delta}$

Magnetic field, $B = \dfrac{0 . 5}{\dfrac{\sqrt{3}}{2}} = \dfrac{\dfrac{1}{2}}{\dfrac{\sqrt{3}}{2}} = \dfrac{1}{\sqrt{3}}G$.