Barron's 2nd Edition DT Q50

Barron's Book

50) The problem asks us to find the equivalent resistance once the switch is closed. This means we have 2 parallels. One of the parallels has a 4 Ohm resistor, while the other parallel has a series of multiple resistors.

Where R is resistance

First, we will find the resistance in each of the two parallels, and then we can find the resistance of the circuit as a whole.

Since the top parallel wire has two resistors in series, the equivalent resistor is just the sum. The bottom parallel wire just has 1 resistor, so that is already the equivalent resistance for that parallel.

Where R is resistance

Now, we can use our parallel circuit resistance formula to find the total resistance of the circuit.

Where R is resistance

Answer: D