12. Trade Commissions

using System;

namespace TradeCommissions
{
    class Program
    {
        static void Main(string[] args)
        {
            string town = Console.ReadLine();
            double sum = double.Parse(Console.ReadLine());

            if (town == "Sofia")
            {

                if (sum > 0 && sum <= 500)
                {
                    Console.WriteLine($"{0.05 * sum:F2}");
                }

                else if ( sum > 500 && sum <= 1000)
                {
                    Console.WriteLine($"{0.07 * sum:F2}");
                }

                else if (sum > 1000 && sum <= 10000)
                {
                    Console.WriteLine($"{0.08 * sum:F2}");
                }

                else if (sum > 10000)
                {
                    Console.WriteLine($"{0.12 * sum:F2}");
                }

                else
                {
                    Console.WriteLine("error");
                }
            }

            if (town == "Varna")
            {

                if (sum > 0 && sum <= 500)
                {
                    Console.WriteLine($"{0.045 * sum:F2}");
                }

                else if (sum > 500 && sum <= 1000)
                {
                    Console.WriteLine($"{0.075 * sum:F2}");
                }

                else if (sum > 1000 && sum <= 10000)
                {
                    Console.WriteLine($"{0.10 * sum:F2}");
                }

                else if (sum > 10000)
                {
                    Console.WriteLine($"{0.13 * sum:F2}");
                }

                else
                {
                    Console.WriteLine("error");
                }
            }
            if (town == "Plovdiv")
            {

                if (sum > 0 && sum <= 500)
                {
                    Console.WriteLine($"{0.055 * sum:F2}");
                }

                else if (sum > 500 && sum <= 1000)
                {
                    Console.WriteLine($"{0.08 * sum:F2}");
                }

                else if (sum > 1000 && sum <= 10000)
                {
                    Console.WriteLine($"{0.12 * sum:F2}");
                }

                else if (sum > 10000)
                {
                    Console.WriteLine($"{0.145 * sum:F2}");
                }

                else
                {
                    Console.WriteLine("error");
                }
            }
            else if(town != "Sofia" && town != "Varna" && town != "Plovdiv")
            {
                Console.WriteLine("error");
            }
        }

    }
}

РЕШЕНИЕ С ТЕРНАРЕН ОПЕРАТОР:

using System;

namespace TradeCommissions
{
    internal class Program
    {
        static void Main(string[] args)
        {

            string town = Console.ReadLine();
            double sum = double.Parse(Console.ReadLine());

            double commission =
                ( sum > 0 && sum <= 500 ? (town == "Sofia" ? 0.05 : town == "Varna" ? 0.045 : town == "Plovdiv" ? 0.055 : 0)
                : sum > 500 && sum <= 1000 ? (town == "Sofia" ? 0.07 : town == "Varna" ? 0.075 : town == "Plovdiv" ? 0.08 : 0)
                : sum > 1000 && sum <= 10000 ? (town == "Sofia" ? 0.08 : town == "Varna" ? 0.1 : town == "Plovdiv" ? 0.12 : 0)
                : sum > 10000 ? (town == "Sofia" ? 0.12 : town == "Varna" ? 0.13 : town == "Plovdiv" ? 0.145 : 0) : 0) * sum;

            Console.WriteLine(commission == 0 ? "error" : $"{commission:F2}");
        }
    }
}