you are asking to execute a string or varchar2 as equation, the result is a value of this equation. right?
here is a solution for this, I assume the space in "7 4/5" means "7+ 4/5" or you have to modify the function bellow to meet your requirements.
create or replace function get_equation_val( p_equation varchar2) return number is
eq varchar2(500);
eq_stmt varchar2(500);
eq_result number;
begin
eq := replace(p_equation, ' ', '+');
eq_stmt := 'select ' || eq || ' from dual';
execute immediate eq_stmt
into eq_result;
-- dbms_output.put_line(eq_result);
return(eq_result);
end get_equation_val;
and here is a test for this function:
select get_equation_val('7 4/5') from dual;