After visiting 10 homes he has 5*5 cookies=25 cookies*78 cals=1950 cals. But he visited 11 homes. Did he eat any cookies at the 11th home? We know he only has 5 cookies after visiting two homes, so we only know for certain that he had 1950 calories. We cannot say he took 2 cookies or 3 cookies from the 11th home and we can't assume he broke any cookies in half, because there's no more information. We can only work off information that we're sure of. So 1950 cals has to be the answer.