question: Mel uses a 900-watt air conditioner for 8 hours a day. This means that each hour the AC uses 900 watts of energy. If he reduces the time he uses the air conditioner by 5 hours a day, how many kilowatts of electric energy will he save in 30 days?
answer: An air conditioner uses 900 x 8 = <<900*8=7200>>7200 watts for 8 hours a day.
  An air conditioner uses 900 x 5 = <<900*5=4500>>4500 watts for 5 hours a day.
  So, Mel saves 7200 - 4500 = <<7200-4500=2700>>2700 watts per day.
  That is 2700/1000 = <<2700/1000=2.7>>2.7 kilowatts per day since 1 kilowatt is equal to 1000 watts.
  Hence, in 30 days he will have 2.7 x 30 = <<2.7*30=81>>81 kilowatts of electric energy saved.
  #### 81