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.
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