Resistors in series
When resistors are connected in series that means that they are attached end to end and the same electric current flows through all of them. Although they all passed the same current the potential difference, that is the voltage, across each of the resistors is likely to be different.
The photograph shows measurement of the total of two resistors connected in series.
Measuring the total of two resistors
Resistance is calculated from the potential difference and the current. The resistance equals potential difference divided by current.
Calculating the total resistance is straightforward, it is just the total, that is the sum, of each of the resistors. Each resistance is nominally 100 ohms but the actual resistance of each is slightly less. The totals do not exactly match but they are very close; there is always some experimental error.
The first part video lesson below explains these points: