A60-watt light bulb is connected to a circuit using 120 volts. what is the resistance in the circuit?
a. 7,200 ohms
b. 240 ohms
c. 2 ohms
d. 0.5 ohms

There are a few different formulas for electrical power.  They involve
different combinations of voltage, current, and resistance.

        ==>         Power  =  (current)²  x  (resistance)

        ==>         Power  =  (voltage)²  /  (resistance)

        ==>         Power  =  (voltage)  x  (current) .

In this case right now, we know the power and voltage, and
we need to find the resistance.  So maybe it's a good idea
to use the formula that involves power, voltage and resistance.

        ==>         Power  =  (voltage)²  /  (resistance)

Multiply each side
by  'resistance':          Power x resistance = voltage²

Divide each side
by  'power':                Resistance  =  voltage² / power

                                                 = (120 volts)² / (60 watts)

                                                 =  (14,400 / 60)    volts²/watts

                                                 =      240 ohms  . 

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