title: Textbox only counts for one variable instead of summing both up
body: <p>I am creating a program that calculates the user's electricity bill. However, there's an issue, I only manage to calculate them one by one, but when I try to sum them up, it didn't work, it only displays the total of one variable instead of summing both up.</p>
  <p><a href="https://i.stack.imgur.com/WwLuC.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/WwLuC.png" alt="Image" /></a></p>
  <p>Here's the code, it's not the full code as it is very long since i need to calculate March to December, so i only took march and april.</p>
  <p>(This is to determine price based on usage)</p>
  <p><em>The service tax is 6%</em>, I've already put it in the service tax variable.</p>
  <pre><code>    Select Case True
          Case mar &gt;= 1 To mar &lt;= 200
              unitsConsumedMar = mar
              marTotalAmount = (unitsConsumedMar * 0.109)
              serviceTax = marTotalAmount * serviceTax
              marTotalAmount += serviceTax
              TextBox16.Text = marTotalAmount.ToString(&quot;C2&quot;)
  
  
          Case apr &gt;= 1 To apr &lt;= 200
              unitsConsumedApr = apr
              aprTotalAmount = (unitsConsumedApr * 0.109)
              serviceTax = aprTotalAmount * serviceTax
              aprTotalAmount += serviceTax
              TextBox16.Text = aprTotalAmount.ToString(&quot;C2&quot;)
          End Select
  
      Select Case True
  
          Case mar &gt; &quot;200&quot; And mar &lt;= &quot;300&quot;
              unitsConsumedMar = mar
              marTotalAmount = (unitsConsumedMar * 0.153)
              serviceTax = marTotalAmount * serviceTax
              marTotalAmount += serviceTax
              TextBox16.Text = marTotalAmount.ToString(&quot;C2&quot;)
  
  
          Case apr &gt; &quot;200&quot; And apr &lt;= &quot;300&quot;
              unitsConsumedApr = apr
              aprTotalAmount = (unitsConsumedApr * 0.153)
              serviceTax = aprTotalAmount * serviceTax
              aprTotalAmount += serviceTax
              TextBox16.Text = aprTotalAmount.ToString(&quot;C2&quot;)
          End Select
  
        totalBill = Val(marTotalAmount) + Val(aprTotalAmount)
        textbox20.Text = totalBill.ToString(&quot;C2&quot;)
  </code></pre>
  <p>These all work if March is 200 and April is 201, but then textbox16 returns some weird amount, it looks like it sums up both the amount from march and april instead of adding it up to textbox20.</p>
  <p><a href="https://i.stack.imgur.com/3siwu.png" rel="nofollow noreferrer">Image</a></p>
  <p>for some reason, if i put 200 in one textbox and 201 in another, it calculates correctly since 70.98 + 23.11 is 94.09, but the monthly total amount shouldn't be 70.98, since 23.11 + 32.60 is not even anywhere near 70...</p>