The two main ways of increasing the current in an electrical circuit are by increasing the voltage or by decreasing the resistance.
If you increase the voltage across a component, there will be more current in the component. Too high a voltage and the bulb will blow.
If you increase the number of lamps in a series circuit, there will be less current. The lamps resist the current, so if you put more lamps into the circuit, there is more resistance.
You could do the same with a variable resistor.
The quantities voltage, current and resistance are linked by the relationship:
If there is more than one voltage or current, you use the voltage across the resistor and the current through it, not just any values that you see in the question.
To calculate the resistance you need to take the following steps:
Click to listen to the Naked Scientists' explanation of electrical resistance.