losing precision while calculating cost - vb.net

Question

I've got this program to calculate the cost of sheets of paper. First the prices and bundles are declared and intialized. Then I use if else statements with division and modulos to break down the bundle into charging rates. Although my paper bundle amounts are correctly computed, my total costs are off. Can anyone spot what I'm doing wrong here?
Module Paper
Sub Main()
'declare variables
Dim PaperAmountReq As Double
Dim PricePer_1_Sheet As Double = 0.1
Dim PricePer_100_Sheets As Double = 0.055
Dim PricePer_500_Sheets As Double = 0.04
Dim PricePer_1000_Sheets As Double = 0.03
Dim NumberOfSingles As Double = 0
Dim NumberOf100s As Double = 0
Dim NumberOf500s As Double = 0
Dim Numberof1000s As Double = 0
Dim TotalCost As Double
Console.Write("Enter number of sheets of paper needed: ")
PaperAmountReq = Console.Readline
If PaperAmountReq / 1000 >= 1 Then
Numberof1000s = PaperAmountReq \ 1000
PaperAmountReq = PaperAmountReq Mod 1000
End If
If PaperAmountReq / 500 >= 1 Then
NumberOf500s = PaperAmountReq \ 500
PaperAmountReq = PaperAmountReq Mod 500
End If
If PaperAmountReq / 100 >= 1 Then
Numberof100s = PaperAmountReq \ 100
PaperAmountReq = PaperAmountReq Mod 100
End If
If PaperAmountReq / 1 = PaperAmountReq Then
NumberOfSingles = PaperAmountReq
End If
'TotalCost = (Convert.ToDouble(Numberof1000s) * PricePer_1000_Sheets) + (Convert.ToDouble(NumberOf500s) * PricePer_500_Sheets) + (Convert.ToDouble(Numberof100s) * PricePer_100_Sheets) + (Convert.ToDouble(NumberOfSingles) * PricePer_1_Sheet)
TotalCost = (Numberof1000s * PricePer_1000_Sheets) + (NumberOf500s * PricePer_500_Sheets) + (Numberof100s * PricePer_100_Sheets) + (NumberOfSingles * PricePer_1_Sheet)
Console.WriteLine("Number of 1000 packages: " & Numberof1000s)
Console.WriteLine("Number of 500 packages: " & Numberof500s)
Console.WriteLine("Number of 100 packages: " & Numberof100s)
Console.WriteLine("Number of singles packages: " & NumberOfSingles)
Console.Write("Total Cost: " & TotalCost)
End Sub
End Module

Answer 1

With this:
If PaperAmountReq / 1000 >= 1 Then
Numberof1000s = PaperAmountReq \ 1000
PaperAmountReq = PaperAmountReq Mod 1000
End If
if you have 1500, you are setting Numberof1000s equal to 1 - not 1000. So when you calculate your total cost, your value of (presumably) cost/sheet is being multiplied by 1 instead of 1000.
Either change your equation here
TotalCost = (Numberof1000s * PricePer_1000_Sheets) + (NumberOf500s * PricePer_500_Sheets) + (Numberof100s * PricePer_100_Sheets) + (NumberOfSingles * PricePer_1_Sheet)
to be something like
TotalCost = (Numberof1000s * PricePer_1000_Sheets * 1000) + (NumberOf500s * PricePer_500_Sheets * 500) + (Numberof100s * PricePer_100_Sheets * 100) + (NumberOfSingles * PricePer_1_Sheet * 1)
or adjust the PricePer_XXX_Sheets values to be the equivalent values.
Dim PricePer_1_Sheet As Double = 0.1
Dim PricePer_100_Sheets As Double = 0.055 * 100
Dim PricePer_500_Sheets As Double = 0.04 * 500
Dim PricePer_1000_Sheets As Double = 0.03 * 1000

Answer 2

Well your numberof variables should be integers or maybe bigintegers if not big enough as should PaperAmountReq
Your code seems to be assuming integer division
in these segments
If PaperAmountReq / 500 >= 1 Then
NumberOf500s = PaperAmountReq \ 500
PaperAmountReq = PaperAmountReq Mod 500
End if
in actual fact because you are using doubles, it's doing floating point division.
I'd also recommend that you use decimal instead of double for your price? and Total Cost variables.
Doubles are very precise, but often very inaccurate.

Answer 3

Looks like you need to convert the NumberOfXXXs variables to sheets, rather than bundles.
Your formula at the end uses per-sheet costs with per-bundle numbers. Either update the costs to be per-bundle, or update the bundle numbers to equal the round number of sheets in that bundle:
TotalCost = NumberOf1000s * 1000 * PricePer_1000_Sheets

Answer 4

Dim PricePer_100_Sheets As Double = 0.055
Dim PricePer_500_Sheets As Double = 0.04
Dim PricePer_1000_Sheets As Double = 0.03
should be
Dim PricePer_100_Sheets As Double = 0.055 * 100
Dim PricePer_500_Sheets As Double = 0.04 * 500
Dim PricePer_1000_Sheets As Double = 0.03 * 1000