What is the approximate total resistance of a 100- and a 200-ohm resistor in parallel?
Why Resistors in parallel combine as the product over the sum: (100 x 200) / (100 + 200) = 20000 / 300 = 66.7 ohms, which rounds to about 67 ohms. The total in a parallel network is always smaller than the smallest resistor present, because you are adding more paths for current. Here 67 ohms is indeed below the 100-ohm branch.
Watch out The 300-ohm choice is what you would get by adding them in series, and 150 ohms is just the average of the two values, which is not how parallel resistance works.
Parallel = product over sum, and the answer must be less than the smallest resistor.