title: How to subtract values from textbox in form from Field in table (Access '07 VBA)?
body: <p>I am developing a canteen transaction system in MS Access 2007. In my "Add Food Item:" form, I have a series of textboxes and comboboxes, a button, "Add Item" and a listbox. The names of text and combo boxes are as follows :</p>
  
  <p>Textbox names:
  txtRate,
  txtQty</p>
  
  <p>Combobox names:
  cbxICode,
  cbxItems</p>
  
  <p>On the click of the Add Item button, the selected item with its quantity, rate etc. gets tranferred to the listbox. (I asked asked the transfer question <a href="https://stackoverflow.com/questions/15730416/add-values-to-a-multi-column-listbox-in-access-07-vba">here</a>)</p>
  
  <p>Now, in my tblItems, there is a "QtyAvailable" field which indicates the available quantity of the currently existing food items. How do I:</p>
  
  <ul>
  <li>Subtract the quantity of a food item the user enters from the QtyAvailable field in tblSampleData?</li>
  </ul>
  
  <p>Advanced help is appreciated.</p>
  
  <p>Thanks,</p>
  
  <p>S Sandeep</p>