Resistors are said to be connected in “Series“, when they are daisy chained together in a single line. Since all the current flowing through the first resistor has no other way to go it must also pass through the second resistor and the third and so on. Resistors in series have a Common Current flowing through them as the current that flows through one resistor must also flow through the others as it can only take one path. Then: IR1 = IR2 = IR3 = IR4 etc.
I=VS/RT = VR1/R1 = VR2/R2 = VR3/R3
Voltage Divider Networks
VOUT=VS R2/RT = (VS R2)/(R1+R2)