var results = 
    from e in exposureGrouping
    group e by new { e.Category, e.Area } into g
    orderby g.Key.Category, g.Key.Area
    select new WorldwideExposure
    {
        Category = g.Key.Category,
        Area = g.Key.Area,
        Open_Market = g.Where(x => x.ClassCode == "OM").Sum(x => x.Usd_Gross_Exposure),
        North_American_Binders = g.Where(x => x.ClassCode == "NA").Sum(x => x.Usd_Gross_Exposure),
        MI_And_Reo = g.Where(x => x.ClassCode == "MI").Sum(x => x.Usd_Gross_Exposure),
        Other = g.Where(x => x.ClassCode == "OT").Sum(x => x.Usd_Gross_Exposure),
        Total = g.Sum(x => x.Usd_Gross_Exposure)
    };