b
(b)Suppose magnetic field is zero at point \( P.\) Which lies at a distance \(x\) from \(10\) unit pole. Hence at \(P\)
\(\frac{{{\mu _0}}}{{4\pi }}.\frac{{10}}{{{x^2}}} = \frac{{{\mu _0}}}{{4\pi }}.\frac{{40}}{{{{\left( {30 - x} \right)}^2}}} \Rightarrow x = 10\,cm\)
So from stronger pole distance is \(20 \,cm.\)
