What is the approximate total resistance of a 100- and a 200-ohm resistor in parallel?

The formula for two resistors in parallel is Rt = (R1 x R2) / (R1 + R2), which is also known as the 'product over sum' formula. Plugging in the values gives (100 x 200) / (100 + 200). Calculating this out, you get 20000 / 300. This simplifies to 200 / 3, or approximately 66.7 ohms, which rounds to 67 ohms. Remember that total resistance in a parallel circuit is always less than the lowest individual resistor. Since the lowest resistor here is 100 ohms, the answer must be less than 100, which easily rules out 300 ohms and 150 ohms.