question: A tomato vendor decides to switch who he buys his tomatoes for.  He sells 500 tomatoes a day.  He used to buy them for $.5 each but he gets a new vendor who sells them for $.4 each.  How much money does he save a week?
answer: He buys 500*7=<<500*7=3500>>3500
  He used to pay 3500*.5=$<<3500*.5=1750>>1750
  He now pays 3500*.4=$<<3500*.4=1400>>1400
  So he saves 1750-1400=$<<1750-1400=350>>350
  #### 350