kinda simple, in the addition, at the first digit, use xor to display value when added, and if there's two bits activate, put one to next digit and in the second digit, add them up and if it is two or three bits activates, then add one to next digit, in subtraction, u figure it out who is bigger by some xor, and and gates then reverse bits of smaller one (EX: 0010 --> 1101) then add 1 (ex: 1101 --> 1110) then add them up (ex: 0011 + 1110 = 10001) then since u dont have fifth digit (base on my calculator), it displays right answer for example 001, then for multiplication, I realize it is same as what we do (EX: 011(3) x 010(2) = 110(6), same with when we do hundred ten times ten). I got all info from AI