Ellen spent $15.45 on lunch at a restaurant. How much will she pay for a 20% tip? What is her total cost for lunch with the tip? Be sure to give answers to both questions.

Ellen will pay 15.45 * 20/100 = $3.09 for the tip.

Her total cost for lunch including the tip is 15.45 + 3.09 = $18.54. Answer: \boxed{18.54}.