I use greedy to solve this problem.
    First,I scanned all the boxes.If it had N new different coins that Tom wanted.The purchase will added about N*100
    Then,I think Tom would buy this box.And it costs him 100 yen.
    At last.If the purchase>0 ,then I would buy some boxes that Tom did not have been bought.
    I was WA on #1
    Am I wrong?
    At last,I'm sorry for my poor English.
    Thanks a lot.