Is 1010 valid BCD?
coresix wasted patterns per nibble
Answer
invalid — 1010 is one of the six illegal codes
Why this example is worth doing
Four bits have sixteen patterns but decimal needs only ten, so 1010 through 1111 are invalid BCD. The page uses that gap two ways: as the source of BCD's inefficiency — about a sixth of the code space is wasted — and as a free error check, since a nibble in the illegal range means something has gone wrong. It also feeds directly into why BCD addition needs a correction step.
Try your own input in the BCD converter. Pack and unpack binary-coded decimal, and add with the +6 correction shown.
How the answer is reached
Nibble range
A BCD nibble codes 0000…1001. 1010 is 10, outside that range.