feat : add description and c# solution

202508/808 soupServings/C#/Program.cs

@@ -0,0 +1,56 @@
+using System;
+
+public class Solution
+{
+    private Dictionary<(int, int), double> dp = new Dictionary<(int, int), double>();
+    public double SoupServings(int n)
+    {
+        if (n > 5000)
+            return 1.0;
+
+        n = (n + 24) / 25;
+
+        // use dynamic programming
+        return Helper(n, n);
+    }
+
+    private double Helper(int a, int b)
+    {
+        if (a <= 0 && b <= 0)
+        {
+            return 0.5;
+        }
+        if (a <= 0)
+        {
+            return 1.0;
+        }
+        if (b <= 0)
+        {
+            return 0.0;
+        }
+        if (dp.ContainsKey((a, b)))
+        {
+            return dp[(a, b)];
+        }
+
+        double probability = 0.25 * (
+            Helper(a - 4, b) +
+            Helper(a - 3, b - 1) +
+            Helper(a - 2, b - 2) +
+            Helper(a - 1, b - 3)
+        );
+
+        dp[(a, b)] = probability;
+        return probability;
+    }
+}
+
+class Program
+{
+    static void Main()
+    {
+        var Solution = new Solution();
+        var ans = Solution.SoupServings(800);
+        Console.WriteLine(ans);
+    }
+}