Well, for part b, the question is not whether there was more than one error, but rather whether the checksum comes out the same. In this case, it happens to come out the same (because the same position bits were flipped in both 8-bit words), so the error is not detected, but if different bits had been flipped, the checksum would have detected the error. (It's more likely that the checksum would notice a 2-bit error than not, but it's not guaranteed to detect 2-bit errors!)
