c# calculate percent of textbox value

Question

I have 2 textboxes

 

textbox1, textbox2

 

I want to calculate 15% of the number user entered in textbox1 and show it in textbox2

 

I tried this :

 

 

if (!string.IsNullOrEmpty(textBox3.Text))

textBox7.Text = (int.Parse(textBox3.Text) *100 / "15%).ToString();