Multiplying BigInteger by Double

Question

My Physics teacher gave the class a hard task. I'm trying to create a program that will calculate some things for me. At a certain point I need to multiply an amount of molecules by a percentage. Ulong can't hold numbers as large as 6022 * 10 ^ 19, so I have to use BigInteger from .net 4.0. However multiplication cannot be applied to BigInteger and double. I only need the whole number. Is there any way to bypass this? The code is as follows:

private static BigInteger pencapmoles = BigInteger.Pow(6022/9, 19);
private static BigInteger inkmoles = pencapmoles;
private static BigInteger tries = 0;
private static BigInteger molesinbucket = BigInteger.Pow(26761768, 19);
private static double percentage;
static void Main(string[] args)
{
    while (inkmoles > 1) 
    {
        percentage = (double)(inkmoles / molesinbucket);
        inkmoles = pencapmoles * percentage;
        tries++;
    }
    Console.WriteLine($"It only took us {tries} tries.");
}

Answer 1

You would be better off switching all computations either to double or to BigInteger. C# double has enough range to hold values a lot greater than 10¹⁹, but the answer is going to have a slight imprecision. It is a near certainty that it is going to be enough for the purposes of solvind your task.

If you switch to BigInteger, multiply first, and then do the division:

inkmoles = (inkmoles * pencapmoles) / molesinbucket;