Feel free on trying the following LINQ as an alternate
Assign Activity
dt2 =
(From d in dt1.AsEnumerable
let c1 = d("Descr.......").toString.Trim.ToUpper
Let c2 = d("Novo saldo....").toString.Trim
Where c1.Contains("CAR_DUPL") AND CDbl(c2) >= 500
Select r=d).CopyToDataTable
just complete the column name
ensure that the data is within the expected formats and values