Cannot figure out calculation - vb.net

Question

I need to make a phone bill program that would allow the user to calculate their phone bill. They would enter their minutes, texts and data allowance as well as how much they've used.
Each extra minute is 30p, each extra text is 10p and data used over the limit is £10.00, all added to the final cost.
The problem is that I cannot get the program to calculate the cost of each minute, I guess that after finding out how the minutes are calculated I will be able to do the same for the texts and the data.
Thanks for the help, much appreciated.
This is the problem
Public Class Form1
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
End Sub
Private Sub btn1TotalCost_Click(sender As Object, e As EventArgs) Handles btn1TotalCost.Click
Dim MinutesAllowed As Integer
Dim MinutesUsed As Integer
Dim TextsAllowed As Integer
Dim TextsUsed As Integer
Dim DataAllowed As Integer
Dim DataUsed As Integer
Dim MinutesTotalCost As Double
Dim TextsTotalCost As Double
Dim DataTotalCost As Double
Dim MonthlyCost As Double
Dim TotalCost As Double
MinutesAllowed = Val(txtBoxMinutesAllowed.Text)
MinutesUsed = Val(txtBoxMinutesUsed.Text)
TextsAllowed = Val(txtBoxTextsAllowed.Text)
TextsUsed = Val(txtBoxTextsUsed.Text)
DataAllowed = Val(txtBoxDataAllowed.Text)
DataUsed = Val(txtBoxDataUsed.Text)
MinutesTotalCost = Val(txtBoxTotalCost.Text)
TextsTotalCost = Val(txtBoxTotalCost.Text)
DataTotalCost = Val(txtBoxTotalCost.Text)
TotalCost = Val(txtBoxTotalCost.Text)
'Minutes
MinutesAllowed = Integer.Parse(txtBoxMinutesAllowed.Text)
MinutesUsed = Integer.Parse(txtBoxMinutesUsed.Text)
MonthlyCost = Val(txtBoxMonthlyCost.Text)
If MinutesAllowed >= MinutesUsed Then
MinutesTotalCost = 0
Else
MinutesTotalCost = (MinutesUsed - MinutesAllowed) * 0.3
End If
txtBoxTotalCost.Text = CType(MinutesTotalCost + MonthlyCost, String)
'Texts
TextsAllowed = Integer.Parse(txtBoxTextsAllowed.Text)
TextsUsed = Integer.Parse(txtBoxTextsUsed.Text)
MonthlyCost = Val(txtBoxMonthlyCost.Text)
If TextsAllowed >= TextsUsed Then
TextsTotalCost = 0
Else
TextsTotalCost = (TextsUsed - TextsAllowed) * 0.15
End If
txtBoxTotalCost.Text = CType(TextsTotalCost + MonthlyCost, String)
'Data
DataAllowed = Integer.Parse(txtBoxDataAllowed.Text)
DataUsed = Integer.Parse(txtBoxDataUsed.Text)
MonthlyCost = Val(txtBoxMonthlyCost.Text)
If DataAllowed >= DataUsed Then
DataTotalCost = 0
Else
DataTotalCost = (DataUsed - DataAllowed) + 10.0
End If
txtBoxTotalCost.Text = CType(DataTotalCost + MonthlyCost, String)
End Sub
End Class

Answer 1

MinutesTotalCost = MinutesUsed - MinutesAllowed * 0.3
Multiplication and division operators have precedence over the addition and subtraction operators.
You need to add brackets to the calculation.
MinutesTotalCost = (MinutesUsed - MinutesAllowed) * 0.3
You also overwrite the result of the calculation here:
MinutesTotalCost = txtBoxTotalCost.Text
Edit: Working code for just the minutes
Dim MinutesAllowed As Integer
Dim MinutesUsed As Integer
Dim MinutesTotalCost As Double
Dim MonthlyCost As Double
MinutesAllowed = Integer.Parse(txtBoxMinutesAllowed.Text)
MinutesUsed = Integer.Parse(txtBoxMinutesUsed.Text)
MonthlyCost = Val(txtBoxMonthlyCost.Text)
If MinutesAllowed >= MinutesUsed Then
MinutesTotalCost = 0
Else
MinutesTotalCost = (MinutesUsed - MinutesAllowed) * 0.3
End If
txtBoxTotalCost.Text = CType(MinutesTotalCost + MonthlyCost, String)
Val() returns a double so you need to use Integer.Parse() for int.
Double needs to be converted to string for the textbox.
Edit 2:
Since MonthlyCostdoesn't change you need the line
MonthlyCost = Val(txtBoxMonthlyCost.Text)
only once at the beginning.
You are always overwriting the value of txtBoxTotalCost. Just make a single calculation with all costs i.e.
txtBoxTotalCost.Text = CType(MinutesTotalCost + TextsTotalCost + DataTotalCost + MonthlyCost, String)