C# Floating point problem

you must use rounding

static void Main(string[] args)
        {
            Decimal d = 4.0m / 3.0m;
            Console.Write(System.Math.Round((4m / d),2) == (3m));
            Console.ReadLine();
        }

You're dealing with decimals here
Just round off your final result before comparing

Console.Write(Math.Round((4m / d), 2, MidpointRounding.AwayFromZero) == (3m));

remember, its not a good idea to round off at every point of your calculation (or even when storing it for that matter). Round off when considering it (for display or comparison)

Hope this is what y'r looking for

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            Decimal d = 4.0m / 3.0m;            //same problem in Double
            Console.Write((4m / d) == (3m));    //same problem in this code Console.Write((4m / d).Equals(3m));
            Console.ReadLine();
            //expected output : True
            //actual output : False
        }
    }
}

I underlined your error. It should be:

//expected output : False

And now everything is ok.
Or, maybe, you should learn some concepts first. Try here[^].