To find the current running through the 120-ohm resistor, we need to first calculate the total resistance of the parallel circuit.

The formula for the total resistance of resistors in parallel is:

1/Total Resistance = 1/R1 + 1/R2 + 1/R3 + ...

In this case, we have three resistors: 120 ohms, 60 ohms, and 40 ohms.

So, the total resistance is:

1/Total Resistance = 1/120 + 1/60 + 1/40

1/Total Resistance = (5 + 10 + 15) / 1200

1/Total Resistance = 30 / 1200

Total Resistance = 40 ohms

Now that we know the total resistance of the circuit, we can use Ohm's Law to find the current running through the 120-ohm resistor.

The formula for Ohm's Law is:

Current = Voltage / Resistance

In this case, the voltage of the power source is 12 volts and the total resistance of the circuit is 40 ohms.

So, the current running through the 120-ohm resistor is:

Current = 12 volts / 120 ohms

Current = 0.1 amps

Therefore, the current running through the 120-ohm resistor is 0.1 amps.