Calculate sum(14.50) and 5.0 values the display in text box

Question

Dear Sir 

Please Find the attachment

 

in that  form VAT Percentage and Vat Amount

VAT percentage has different type of percentege like 14.5,5.0  those details are will display in gridvie one by one so  i want to calculate total value of 14.50 and same as 5.0

values from gridview

 

please help me i am new to C#