Your question wasn't very clear, but I think I understand what you want. Additionally, you should really state what language you're working with. Here it is in C#, and shouldn't be too much of a hassle to translate in to other languages.
Console.Write("Enter payment: ");
float payment;
if (float.TryParse(Console.ReadLine(), out payment)) Console.WriteLine((Math.Floor(payment * 100) / 100) * 0.15, + " at 15% tip."); else Console.WriteLine("Invalid input.");
