C++ :

#include <stdio.h>
 
int main(){
    int i,flag,d;
    char str[50];
    float sum;
    while(gets(str) != NULL) {
        i = 0;
        d = 0;
        sum = 0;
        flag = 0;
        while(str[i] != '\0') {
            switch(str[i]) {
                case 'A' : sum += 4; break;
                case 'B' : sum += 3; break;
                case 'C' : sum += 2; break;
                case 'D' : sum += 1; break;
                case 'F' : break;
                case ' ' : --d; break;
                default  : flag = 1; printf("Unknown\n"); break;
            }
            if(flag == 1) break;
            d++; i++;
        }
        if(flag == 0) {
            if(d != 0) printf("%.2f\n",sum/d);
            else printf("0.00\n");
        }
    }
    return 0;
}

Pascal :

var
k,c:string;
i,j:integer;
sum:real;
begin
     while not eoln do
     begin
     c:=' ';
     sum:=0;
     i:=0;
     readln(c);
     for j:=1 to length(c) do
     begin
     k:=copy(c,j,1);
     if k[1]='A' then
     begin
     sum:=sum+4;
     inc(i);
     end
     else if k[1]='B' then
     begin
     sum:=sum+3;
     inc(i);
     end
     else if k[1]='C' then
     begin
     sum:=sum+2;
     inc(i);
     end
     else if k[1]='D' then
     begin
     sum:=sum+1;
     inc(i);
     end
     else if k[1]='F' then
     inc(i)
     else if ord(k[1])=32 then
     else
     begin
     sum:=-1;
     break;
     end; 
     end;
     if i=0 then i:=1;
     if sum=-1 then writeln('Unknow') else writeln(sum/i:0:2);
      end;
     end.