If the Wheatstone bridge with four resistors $R_1, R_2, R_3$ and $R_4$ is balanced,then the correct expression is

Calculate the net resistance of the circuit between $A$ and $B$.

If the Wheatstone bridge shown in the figure is balanced,then the values of $R_1$ and $R_2$ are ...... $\Omega$ and ...... $\Omega$ respectively.

The current in the given circuit is ................ $A$.

$A$ Wheatstone's bridge is balanced with a resistance of $625\, \Omega$ in the third arm,where $P, Q$ and $S$ are in the $1^{st}, 2^{nd}$ and $4^{th}$ arm respectively. If $P$ and $Q$ are interchanged,the resistance in the third arm has to be increased by $51\,\Omega$ to secure balance. The unknown resistance in the fourth arm is ............. $\Omega$.

In the Wheatstone's bridge (shown in figure) $X = Y$ and $A > B$. The direction of the current between $a$ and $b$ will be