You are unlikely to encounter a circuit with just one power supply and two resistors in your exam. However, you can simplify any series resistors to just one combined resistor within a more complicated circuit.
Remember that you can annotate a circuit diagram (or sketch simplified versions) as you go through a problem.
Summary
The current in a series section of a circuit is constant. Therefore, the current through each series resistor is the same. The potential differences across the resistors sum: \(V_T=V_1+V_2+...\).
This means that the resistance of the combination is the sum of the individual resistors:
\(R_T=R_1+R_2+...\)
NB: Many students believe that current should get 'used up' around the circuit. How could you amend this statement?
Current is the same throughout a series circuit as the number of charges passing each point every second is constant (remember that charges experience a force when acted upon by an electric force). The electrical energy decreases around the circuit so that all has been exactly converted into other forms before a charge returns to the power supply.