A) \[10\Omega \]
B) \[15\Omega \]
C) \[20\Omega \]
D) \[25\Omega \]
Correct Answer: B
Solution :
[b] Case ? 1 \[\frac{5}{R}=\frac{{{\ell }_{1}}}{100-{{\ell }_{1}}}.....(1)\] Case ? 2 when R is shunted with an equal resistance R, shunt placed parallel to R. \[\frac{5}{{}^{R}/{}_{2}}=\frac{1.6{{\ell }_{1}}}{100-1.6{{\ell }_{1}}}.....(2)\] By (1) and (2) \[2\,[100-1.6{{\ell }_{1}}]=1.6\,[100-{{\ell }_{1}}]\] \[200-3.2{{\ell }_{1}}=160-1.6{{\ell }_{1}}\] \[40=1.6{{\ell }_{1}}\] \[{{\ell }_{1}}=25cm\] Put this in (1) \[\frac{5}{R}=\frac{25}{75}\] \[R=15\Omega \]
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