To calculate the current running through the 40-ohm resistor, we first need to find the total resistance of the parallel circuit. The formula for the total resistance of resistors in parallel is:

1 / Total Resistance = 1 / Resistance1 + 1 / Resistance2 + 1 / Resistance3

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 = 1200 / 30

Total Resistance = 40 ohms

Now that we know the total resistance of the circuit is 40 ohms, we can use Ohm's Law to calculate the current running through the 40-ohm resistor. Ohm's Law states that the current (I) flowing through a conductor is directly proportional to the voltage (V) applied across the conductor and inversely proportional to the resistance (R) of the conductor. The formula for Ohm's Law is:

I = V / R

In this case, the voltage applied across the circuit is 120 volts, and the total resistance of the circuit is 40 ohms. So, the current running through the 40-ohm resistor is:

I = 120 V / 40 ohms

I = 3 amps

Therefore, the current running through the 40-ohm resistor is 3 amps.