In a meter bridge experiment null point is obtained at 20 cm from one end when resistance X is balanced against resistance Y, where X < Y. Then, the new position of the null point from the same end if X is replaced by 4X will be

50 cm

The correct answer is Option (1) → 50 cm

formula for Null point,

$\frac{x}{y}=\frac{l}{100-l}$

Case 1: $\frac{x}{y}=\frac{20}{100-20}=\frac{1}{4}$

Case 2: $x=4x$

$\frac{4x}{y}=\frac{l'}{(100-l')}=4(\frac{x}{y})=\frac{1}{4}$

$⇒\frac{l'}{100-l'}=1$

$⇒l'=50cm$