LinqPad

//

var q = (from erm in Examreportmappings
                         join es in Examschedules on erm.Erm_id equals es.Erm_id
join rc in Reportcards on es.Subject_id equals rc.Subject_id
                          where  rc.Student_id==1 && erm.Erm_institution_id==1 && erm.Erm_class_id==1 && erm.Erm_section_id==1
                          select new
                         {
                            rc.Subject_id,
es.Subject_name,
rc.Marks,
}).Distinct();
// var total = q.Sum(rc=>rc.Marks);

//var average=q.Average(rc=>rc.Marks);

// var max=q.Max(rc=>rc.Marks);

var min=q.Min(rc=>rc.Marks);

min.Dump();

//