Why can't 1111 be used in a BCD number?

1111 can't be used for Binary Coded Decimal (BCD) because 1111=15 which is made of 2 digits 1 and 5. In BCD a 4-digit binary number is used for every decimal digit. ex. 1111 is incorrect 1 = 0001 5 = 0101 Answer: 0001 0101