Four resistors are connected one after the other so that their resistances add up to an equivalent resistance of 150 ohm. Three of them are R1 = 10 ohm, R2 = 25 ohm, and R4 = 15 ohm. What is the fourth resistance, R3?

1. For resistors connected one after the other, the equivalent resistance is the sum of all four resistances: R_equiv = R1 + R2 + R3 + R4. 2. Rearrange to solve for the unknown: R3 = R_equiv - R1 - R2 - R4. 3. Substitute the known values with SI units: R3 = 150 ohm - 10 ohm - 25 ohm - 15 ohm. 4. R3 = 100 ohm. 5. 50 ohm would result from subtracting only two of the three known resistors instead of all three. 6. 150 ohm would result from forgetting to subtract any of the known resistors at all. _Source: OpenStax Physics (CC BY 4.0), Ch 19 "Electrical Circuits", section Determine the Unknown Resistance_