Hi Vinoth
I think you specified field plot Area field type is in Text Format.
If it is correct try below.
Create One Formula Field. Example: Test
Type tonumber (Field (Ex: U_Land Area)
Click Save and Close
Then
Create another formula Field (TEST1)
Type SUM (TEST)
Click SAVE and Close
Drag the Test 1 in Report
With Regards
Balaji Sampath