What are the uses of a Wheatstone bridge?

$A$ Wheatstone bridge has the resistances $10 \Omega, 10 \Omega, 10 \Omega$ and $30 \Omega$ in its four arms. What resistance joined in parallel to the $30 \Omega$ resistance will bring it to the balanced condition (in $Omega$)?

Five resistances are connected as shown in the figure. The effective resistance between points $A$ and $B$ is

$A$ Wheatstone bridge is used to determine the value of an unknown resistance $X$ by adjusting the variable resistance $Y$ as shown in the figure. For the most precise measurement of $X$,the resistances $P$ and $Q$:

Consider the circuit shown. If all the cells have negligible internal resistance,what will be the current through the $2 \Omega$ resistor when steady state is reached (in $A$)?

In the circuit shown in the given figure,the resistances $R_1$ and $R_2$ are respectively: