Input Box for an excel macro?

Alright, I have an extra credit project due at 2:30pm, and I am stuck on it.

www.cse.psu.edu/~hansford/extraCreditProject.htm

I have this code so far, i just can't finish it
Sub InvestmentCalc()
'
' InvestmentCalc Macro
' Macro recorded 4/13/2005 by jdh336
'
'
Dim rng
strRng = InputBox("Enter Last Cell To Be Filled")
Range("G3:G10").Select
ActiveCell.FormulaR1C1 = "=RC[-2]*RC[-1]"
Range("G3:g10").Select
Selection.AutoFill Destination:=Range("G3:G6"), Type:=xlFillDefault
Range("G3:strRng").Select
End Sub

I am getting a 1004 error telling me that "autofill method of range class failed"

Can anyone help me?
:cry:

not sure if you still need this..

VB Code:

Sub InvestmentCalc()
 
' InvestmentCalc Macro
' Macro recorded 4/13/2005 by jdh336
 
Dim rng
 
strRng = InputBox("Enter Last Cell To Be Filled")
Range("G3:G10").Select
ActiveCell.FormulaR1C1 = "=RC[-2]*RC[-1]"
Range("G3:g10").Select
Selection.AutoFill Destination:=Range("G3:G6"), Type:=xlFillDefault
'you were placing the variable within string quotation marks.
Range("G3:" & strRng).Select
End Sub