Retain A Value From A Textbox And Add To It?

My assignment is to create an application that allows the user to type an item's price into a single textbox, add the price to the previously entered prices, and display the total charge which includes a $15 shipping charge and a 3% sales tax.Example: I type 10 in the textbox and click calculate. the subtotal (before tax and shipping) reads $10. Next, I type 20 in the textbox and click calculate. The subtotal should now read $30.I have everything working fine except adding the different prices. I cannot figure out how to retain the value of the subtotal and add it to the next value entered.