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<div ng-app ng-init="qty=1;cost=2">
  <b>청구서:</b>
  <div>
    수량: <input type="number" min="0" ng-model="qty">
  </div>
  <div>
    금액: <input type="number" min="0" ng-model="cost">
  </div>
  <div>
    <b>함계:</b> {{qty * cost | currency}}
  </div>
  <div>
    <b>합계:</b> {{ qty }} * {{ cost }} = {{ qty * cost | currency}}
  </div>
</div>