The power dissipated by the whole string of tree lights is:
[tex]P= VI[/tex]
where
V is the potential difference of the whole circuit
I is the current flowing through the tree lights.
By using P=68 W and V=120 V, we find the current:
[tex]I= \frac{P}{V}= \frac{68 W}{120 V}=0.57 A [/tex]
Ohm's law for the whole string of tree lights is:
[tex]V=IR_{eq}[/tex]
where [tex]R_{Eq}[/tex] is their equivalent resistance. Re-arranging the equation, we find the value of the equivalent resistance:
[tex]R_{Eq}= \frac{V}{I}= \frac{120 V}{0.57 A}=210.5 \Omega [/tex]
