For what value of $R$ will the net resistance of the circuit be $18 \, \Omega$?

$A$ wire of resistance $12 \, \Omega/m$ is bent to form a complete circle of radius $10 \, cm$. The resistance between its two diametrically opposite points, $A$ and $B$, as shown in the figure, is:

Two resistors having equal resistances are joined in series and a current is passed through the combination. Neglect any variation in resistance as the temperature changes. In a given time interval,

$A$ wire of resistance $R$ and length $L$ is cut into $5$ equal parts. If these parts are joined in parallel,then the resultant resistance will be:

Three unequal resistances are connected in parallel. Two of these resistances are in the ratio $1:2$. The equivalent resistance of these three resistors connected in parallel is $1 \Omega$. What is the highest resistance value among these three resistances if no resistance is fractional (in $\Omega$)?

The resistance between points $A$ and $C$ in the given network is