What is the 1's complement of 0000 1111 0010 1101 number?

The correct answer is B, 1111 0000 1101 0010. The ones complement of a binary number is found by simply inverting every bit, turning each 0 into 1 and each 1 into 0. Taking the given number group by group, 0000 becomes 1111, 1111 becomes 0000, 0010 becomes 1101 and 1101 becomes 0010, which gives 1111 0000 1101 0010. Computers use this idea because subtraction can then be done with addition: the twos complement, used to store negative numbers, is the ones complement plus one. A is wrong because several of its bits are not the inverse of the matching bits. C is wrong because its last eight bits repeat the original number instead of inverting it. D is wrong because its first group should be 1111, not 1001. Exam tip: ones complement means invert every bit; twos complement means invert every bit and add 1.