The equivalent resistance of resistors connected in series is always

An electric kettle has two heating coils. When one of the coils is connected to an a.c. source,the water in the kettle boils in $10$ minutes. When the other coil is used,the water boils in $40$ minutes. If both the coils are connected in parallel,the time taken by the same quantity of water to boil will be ...... $min$.

In the given circuit, the current in resistance $R_3$ is: (in $\,A$)

$A$ wire of resistance $R$ is bent to form a square $ABCD$ as shown in the figure. The effective resistance between $E$ and $C$ is ( $E$ is the mid-point of arm $CD$ ).

$A$ wire of resistance $12 \, \Omega$ is bent to form an equilateral triangle. What is the equivalent resistance between any two corners of the triangle?

$A$ straight wire of resistance $18 \Omega$ is bent in the form of an equilateral triangular loop. The effective resistance between any two vertices of the triangle is (in $Omega$)