CHAR type characteristics can not be converted to numbers and sum

I created a report by using free-hand SQL as a data provider in the Business Objects 5.1 enviroment . However, I found that the CHAR type characteristics can not be converted to numbers and sum. For example, the AMT is a measure object with the CHAR type including the two character string: 20 and 15.33 in the report. I would like to get 35.33 with the following function formula

=Sum(To_Number(<AMT>)) .

Actually, I only get the 2015.33 with the above formula. I assumed that the sum formuls herein means the concatenation character string. The AMT type should not be updated in the free-hand SQL as a data provider.

Could you please help me how to obtain the correct adding result ,35.33 in this report ? Many thanks!

Best regard

Agiler