Since the rate of current is constant throughout a series circuit,
the value of 2 amperes (It) is designated as the value of current
through R1, R2, and R3, as shown in the chart below.
Total resistance of the circuit is computed in the following manner:
Rt = Et
= 200 volts
Rt = 100 ohms
Since Rt is equal to 100 ohms and Rt = R1 + R2 + R3, the value of R3,
the value of R3 may be calculated by subtracting the known resistances
This difference is then equal to the value of the unknown resistor.
Rt = R1 + R2 + R3
R3 = Rt - R1 - R2
R3 = 100 ohms - 50 ohms - 15 ohms
R3 = 35 ohms