Vb.net Multiplying textbox with time and combo box value - vb.net

Question

I have a masked textbox with time value ("hh:mm" the duration the task wil take) and a combo box with 1-10.
How do I mutilply the masked textbox with the combo box to get the total dutation it would take in "hh:mm.
I have txttime1 as the first textbox that holds the time. (masked textbox)
Cmb1 with 1-10, this indicates how many times the txttime1 must be multiplied.
Txtdura1 as the answer to the multiplication. Basically the total duration of the event.
Any direction would be great.

Answer 1

Convert the string from the textbox to a TimeSpan. Then change to TotalMinutes which is a Double so you can perform multiplication. Take the result of the multiplication and change it to a TimeSpan .FromMinutes. You can then format the result in various ways.
Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
Dim ts As TimeSpan = TimeSpan.ParseExact(TextBox2.Text, "h\:m", CultureInfo.CurrentCulture)
Dim dblTotalMinutes As Double = ts.TotalMinutes
Dim Multiplier As Integer = CInt(ComboBox1.SelectedItem)
Dim MultipliedMinutes As Double = dblTotalMinutes * Multiplier
Dim MultipliedTimeSpan As TimeSpan = TimeSpan.FromMinutes(MultipliedMinutes)
'For days:hours:minutes
TextBox3.Text = MultipliedTimeSpan.ToString("d\:hh\:mm")
'Total Hours as double
TextBox4.Text = MultipliedTimeSpan.TotalHours.ToString
End Sub

Answer 2

Split the hours and minutes apart from each other, create a TimeSpan object and multiply that by the factor from the ComboBox:
Dim duration As TimeSpan = TimeSpan.Parse(txttime1.Text)
Txtdura1 = duration.Multiply(Integer.Parse(Cmb1.Text)).ToString("hh\:mm")
Relevant documentation:
TimeSpan.Parse()
TimeSpan.Multiply()
TimeSpan.ToString()