Totally missed this somehow (and only noticed it now
), sorry for the late reply! Hopefully, you're still around and this is still a relevant response😞
The 1200 watt microwave only uses 40% of its power - ie. 480 watts = 0.48 kilowatts. It uses this for 45 minutes = 0.75 hours.
A kilowatt-hour is a unit of energy. One kilowatt-hour of energy is 'produced' when one kilowatt of power is applied for one hour.
We multiply the power by the time ie. 0.48 kW * 0.75h = 0.36 kWh. The microwave uses this amount of energy every day for 180 days, so it uses a total of 64.8 kWh of energy over that time, which costs 64.8kWh * $0.25/kWh = $16.20.
If you've worked through a lot of past papers, you could also look for student repositories supplied by your school - they often have past papers from your school and perhaps other schools that your teacher/faculty might have procured. otherwise external books are often a good resources. Redoing questions with different numbers is a bit desperate but is also a good enough way to get even more practice in.