To add a number in macro use %EVAL macro

Example

%Let a= 2;
%Let b= 3;
%Let c= &a + &b ;
then it will give you value

c=2 + 3

but If you will use %eval it will give you sum

%Let c= %eval(&a + &b)
Now this will give you the correct answer

c=5

%macro add(a,b);
%if (%datatyp(&a)=NUMERIC and %datatyp(&b)=NUMERIC) %then %
do;
    %put The result is %sysevalf(&a+&b).;
%end;
%else %do;
   %put Error:  Addition requires numbers.;
%end;
%mend add;

You can invoke the ADD macro as: 

%add(5.1E2,225)