Very basic maths

[Stryker]

if I had a base value, which was marked up by say, 10%, then was marked up again by 15%, to give a final value, what calculation would work to work out the base value if only given the final?

And what would be the best way to set it up to allow for variations in those percentages?

Thanks

[transcendental]

base_value =(100/110)*(100/115)*final_value

[JohnVB6]

Basically, when you increase a number by some percentage you're actually multiplying by 1 + the percentage like this:
A * (1 + some%) = B
If A = 100 and the percent to add = 10% (10%=.1) then
100 * (1 + .1) = B
100 * 1.1 = B
B = 110
so to determine A given B and the percent increase of A you would use:
A = B / (1 + .1)
A = 110 / 1.1
A = 100
This is demonstrated in the following code where the base number is increased by multiple percentages, just place 2 command buttons on a form and paste this code.

Option Explicit
Dim MyBaseNumber, MyNewNumber
Dim PercentArray()

Private Sub Form_Load()
  'change these values to suit your needs
  ReDim PercentArray(2) 'holds percent multipliers (as many as needed)
  MyBaseNumber = 120
  PercentArray(0) = 0.1 '10%
  PercentArray(1) = 0.15 '15%
  PercentArray(2) = 0.05 '05%
End Sub

Private Sub Command1_Click()
  Dim i As Integer
  Dim MsgTemp As String
  MyNewNumber = MyBaseNumber
  'the following loop creates a new number based on the percent array
  For i = 0 To UBound(PercentArray)
    MyNewNumber = MyNewNumber * (1 + PercentArray(i))
    MsgTemp = MsgTemp & " + " & PercentArray(i) * 100 & "%"
  Next
  MsgBox MyBaseNumber & MsgTemp & " = " & MyNewNumber
End Sub

Private Sub Command2_Click()
  Dim i As Integer
  Dim MsgTemp As String
  
  If Len(MyNewNumber) = 0 Then
    MsgBox "Click Command1 first."
    Command1.SetFocus
    Exit Sub
  End If
  MyBaseNumber = MyNewNumber
  'the following loop recreates the base number
  For i = UBound(PercentArray) To 0 Step -1
    MyBaseNumber = MyBaseNumber / (1 + PercentArray(i))
  Next
  MsgBox "The base number is: " & MyBaseNumber
End Sub

[Starman]

This is when you find out what a 'rounding error' is.
We had to try to create pre-tax costs from a group of tax-included figures.
Our accoutant suggested that this is why so many shop prices ended in .99

[galway]

the .99 thing is actually just a marketing scam to make you think it is cheaper. When people se $19.99 flash up as a price it just seems cheaper then $20.00 to most people. It also allows them to say "for under twenty dollars".

Quick brain teaser. What TV character invented the 99 cent coin, but had it rejected by the governemnt beause taxes make it pointless.

[Starman]

>marketing scam
Oh yeah, I know that, our accountant did say that with a smile, but when you try to find what the original cost of several thousand items was when the data you're given contains 17.5% tax (which you know was rounded in the customer's favour), you know that you are only going to get a correct answer 1 in 47 times - 1 (smallest denomination coin) multiplied by 46/47*several thousand items, amounts to a noticeable amount of money.
Luckily we found another feed that gave us the pre-tax costs.