What are the advantages of connecting electrical devices in parallel with the battery instead of connecting them in series?

How can three resistors of resistances $2 \Omega$, $3 \Omega$, and $6 \Omega$ be connected to give a total resistance of  $4 \Omega$?

How can three resistors of resistances $2 \Omega$, $3 \Omega$, and $6 \Omega$ be connected to give a total resistance of $1 \Omega$?

What is the highest total resistance that can be secured by combinations of four coils of resistance $4 \Omega , 8 \Omega , 12 \Omega , 24 \Omega$?

What is the lowest total resistance that can be secured by combinations of four coils of resistance $4 \Omega , 8 \Omega , 12 \Omega , 24 \Omega$?

Compute the heat generated while transferring $96000 \mathrm{coulombs}$ of charge in one hour through a potential difference of $50 \mathrm{V}$.

An electric iron of resistance $20 \Omega$ takes a current of $5 \mathrm{A}$. Calculate the heat developed in $30 \mathrm{s}$.