using System;

using System.Collections.Generic;   

using System.Collections;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var listOfCalculation = Enumerable.Range(1,10).Select(x=>new Calculation

    {

        Id = x,

        ErrorCode = x.ToString(),

        CalculationDetails = Enumerable.Range(x,10).Select(y=>new CalculationDetails{

            Id = y,

            Amount = y,

            Rate = y,

            Category = y%2==0 ? Category.CategoryA:Category.CategoryB

        }).ToList()

    }).ToList();

    var result = listOfCalculation.Max(x=>x.CalculationDetails.Where(c=>c.Category.Equals(Category.CategoryB)).Max(c=>c.Amount));

    Console.WriteLine(result);

    }

}

​

public class Calculation

{

    public int Id;

    public string ErrorCode;

    public ICollection<CalculationDetails> CalculationDetails;

}

​

public class CalculationDetails

{

    public int Id;

    public decimal? Amount;

    public decimal? Rate;

    public string Category;

}

​

public static class Category

{

    public const string CategoryA = "A";

    public const string CategoryB = "B";

}