Hexadecimal is a number system where, instead of the digits 0,1,2,3,4,5,6,7,8,9, you use the digits 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F. All the facts given are completely true in hexadecimal, which is why I said "Solve for D" is meaningless, since technically the real answer is just D=D.