A garage door opener has a power rating of 350 watts. If the door is in operation for 30 seconds, how many joules of energy are used? (Hint: Remember that 1 watt = 1 joule/sec)

Because of the hint we can conclude what equation we need to solve this problem. We have power and duration that means that we need to express energy:

1 joule = 1watt * 1 second

or

E (energy) = P (power) * t (time duration)

E = 350 * 30 = 10500 joules.