What is the difference between series combination and parallel combination of resistances?

Consider the given circuit and find the current flowing in the circuit and potential difference across the $15 \mathrm{\Omega }$ resistor when the circuit is closed.

Two bulbs of $100 \mathrm{W}$ and $40 \mathrm{W}$ are connected in series. The current through the $100 \mathrm{W}$ bulb is $1 \mathrm{A}$. The current through the $40 \mathrm{W}$ bulb will be:

Calculate the equivalent resistance of the following network:

The equivalent resistance between $A$ and $B$ for the mesh shown in the figure is

Six similar bulbs are connected as shown in the figure with a DC source of emf $E$, and zero internal resistance.
The ratio of power consumption by the bulbs when $\left(\mathrm{i}\right)$ all are glowing and $\left(\mathrm{ii}\right)$ in the situation when two from section $A$ and one from section $B$ are glowing will be, 

Calculate the effective resistance across AB.

In the given circuit, the current through $2 \mathrm{\Omega }$ resistor is

The diagram above shows a circuit with the key k open.

 Calculate the resistance of the circuit when the key k is closed.

 Calculate the current drawn from the cell when the key k is closed.

Each resistance in the given cubical network has resistance of $1\mathrm{\Omega }$ and equivalent resistance between $A$ and $B$ is

The diagram above shows a circuit with the key k open. Calculate the resistance of the circuit when the key k is open and calculate the current drawn from the cell when the key k is open.

Two lamps, one rated $100 \mathrm{W}$ at $220 \mathrm{V}$, and the other $60 \mathrm{W}$ at $220 \mathrm{V}$, are connected in parallel to the electric mains supply. What current is drawn from the line if the supply voltage is $220 \mathrm{V}$?