Well first of all, here's something you're going to need:
Power = (energy) / (time) 1 watt = (1 joule) per second
Energy = (power) x (time) 1 joule = (1 watt) x (1 second)
At this point, your progress grinds to a stop, because, technically, the
current alone doesn't tell you the power. In other words, 'Amperes'
alone doesn't give you the 'watts'.
Power (watts) = (amperes) times (volts) .
So you need to either go ask somebody or else assume the voltage
of the outlet that the microwave oven is plugged into. Without it,
the question can't be answered.
I'm going to assume that you live and go to school somewhere in the USA,
Canada, or Mexico. If that's true, then the outlets in your house supply
electrical energy at 120 volts, and everything you plug into them is designed
to run on 120 volts. Now you have enough information to solve the problem.
Power = (15 amperes) x (120 volts) = 1,800 watts
Energy = (power) x (time) =
(1,800 watts) x (20 minutes) x (60 seconds/minute) =
2,160,000 watt-seconds = 2,160,000 joules
