macro math formula dependent on variables

Hello everyone, I need help creating a macro with a math problem in it.

I have variables from drop down lists in C3 and C4 Which is currently a African Sumac and Agave and the count of how many was removed that day to the left of them.

plants removed
Count Type
5 AFRICAN SUMAC
2 AGAVE



Then i have the current inventory below it with the plant type in C8;9;10 and the current count to the left of those

Current Inventory
Count Type
100 AFRICAN SUMAC
50 ARISTOCRAT PEAR
60 AGAVE


Heres my question, is it possible to make a code that subtracts the removed count from the current count dependent on if the plant types match? and since i am using a large drop down list for the removed plant material would it be possible to make the removed count default to 0 at any time that the plant type is changed. The reason for this is that if there is a code to make it subtract the removed from the current i wouldnt want it to subtract automatically if the removed variable changes via the drop down list. does that make sense? Maybe i could use a module with a macro and a button procedure?

I appreciate all and any help. I have also created a small scale sample of would i would like to do... here is the download link.

http://wikisend.com/download/452542/Book1.xls

I think this does what you want for the example. Place in the sheet module - right-click on the sheet tab, View Code, and paste in:

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I've hardcoded all the ranges, but in reality I expect yours will vary.

The Original Code that you gave me worked in my practice workbook. But when i went to go and change the Ranges and Other Values i must have done something Wrong.

My code is as follows

Private Sub Worksheet_Change(ByVal Target As Range)

Dim v

If Intersect(Target, Range("A21:B25")) Is Nothing Then Exit Sub

If Target.Column = 2 Then
Target.Offset(, -2) = 0
Else
v = Application.Match(Target.Offset(, 2), Range("B34:B38"), 0)
If IsNumeric(v) Then
Range("A34:A38").Cells(v) = Range("A34:A38").Cells(v) - Target
End If
End If

End Sub

All of the numerics are in the "A Column" and all variables are in the "B column"
The plants being removed will be in Range A21:B25
And the Inventory is Range A34:A38


It is giving me a error and "Target.Offset(, -2) = 0" is highlighted

Any Idea where i went wrong?
Your original code worked so there has to be something i changed... Right?

If the target column is 2 then offset 2 to the left is column zero which doesn't exist. So, I think it should be an offset of -1.