How to Calculate amount based on time ?

It really depends on how you look at it:

1. You could do an analysis and calculate the interpolated price.
2. You could say that 2.25 days is 2 * 24h plus 1 * 6h (6h/24h == 0.25)
   So the price would be 2 * 200$ + 1 * 20$ == 420$
   

Actually the price of 200$ for 24 hours doesn't make any sense since 8 three hour tickets cost 80$ which is the same as 4 six hour tickets. So instead of buying two 24 hour tickets and one 6 hour ticket just buy 9 six hour tickets and only pay 180$.

Cheers!

Here is a bit of code you so eagerly yearned for:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace CalculateSomeStrangePricingRequirement
{
    class Program
    {
        public static double CalculatePrice(Double numberOfDaysWithFraction, Double priceOf3HourInterval, Double priceOf24HourInterval)
        {
            Double retVal = Double.NaN;

            Double numberOfWholeDays = Math.Floor(numberOfDaysWithFraction);
            Double numberOfThreeHourIntervals = Math.Ceiling((numberOfDaysWithFraction - numberOfWholeDays) * 8);
            retVal = numberOfWholeDays * priceOf24HourInterval + numberOfThreeHourIntervals * priceOf3HourInterval;

            return retVal;
        }

        static void Main(string[] args)
        {
            Double totalCost = CalculatePrice(2.25, 10, 200);
            Console.WriteLine("The total cost for {0} days is {1}$", 2.25, totalCost);
        }
    }
}

The program outputs for 2.25 420$. If you change that to 2.26 another 3 hour interval is started so the price will be 430$.
And by the way, 24h divided by 3 is 8h. Just a little hint in case you were wondering. If you want you can turn the function I wrote into a one liner. Just as an exercise of course.

Have fun!

Manfred