An electric current passes through a long straight wire. At a distance $5\, cm$ from the wire, The magnetic field is $B$. The field at $20\, cm$ from the wire would be
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(b) $B = {10^{ - 7}}\frac{{2i}}{r} \Rightarrow \frac{B}{{B'}} = \frac{{20}}{5} \Rightarrow B' = B/4$
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