One deterrent to burglary is to leave your front porch light on all the time. if your local power utility sells energy at 17¢ per kilowatt-hour, how much will it cost to leave the above lamp on for the whole week?

To be honest with you, there's not enough information here to calculate
the answer, because we don't know the power consumption of the front
porch light.  The best we can do is represent that as a variable ... pick a
catchy name for it, which will show up in the final answer, and then you
can 'plug in' (get it ?) whatever size light bulb you want to use over the
front porch.

OK.  First we have to give it a name.  I could use a single letter, but
that's what everybody else always does.  Remember ... this is going to
represent the power (watts) consumed by the front porch light when it's
turned on.  It's the symbol that means "Front Porch Light Power" wherever
you see it.  I could easily call it x or y or z, but I'm going to call it " FPLP ".

OK.  FPLP = power consumption of the light, in watts .

0.001 FPLP = power consumption of the light, in kilowatts (kW).

(1 week) · (7 day/week) · (24 hour/day) = 168 hours .

Energy = (power) · (time) = (0.001 FPLP) · (168 hr) = 0.168 FPLP kWhours

Cost = ($0.17 per kWh) · (number of kWh) = (2.86¢) · (FPLP) .

There it is !

For an example, if you use a 100-W bulb in the front porch light,
then FPLP=100 W.  If you leave it on 24/7 for a whole week, then
it's going to cost

        (2.86¢) · (FPLP) = (2.86¢) · (100 W)  =  $2.86