Arduino simple resistors in series
Just a math exercise really.

Say you had a 5V source, two Resistors R1 and R2, and an Arduino.
5V - R1 - R2 - Gnd, with R1/R2 junction feeding the Arduino.
R1 is unknown.

Vout is what the Arduino sees. Vout = 5VxR2/(R1+R2)
Vout (R1+R2) = 5VxR2
R1+R2 = 5VxR2/Vout
R1 = 5VxR2/Vout - R2

Vout = ~ 0.00488mV x Analog In reading. (5V/1024)
If you select R2 of say 5K ohm, and you determine Vout to be 3V (analogRead returns 615), then you can calculate R1:
(5V x 5000)/3V- 5000 = 3333 ohm.

Checking with Vout = 5VxR2/(R1+R2), 5V x 5000/(3333 + 5000) = 3.000V