Question

Think of a number. Double it. Add six. Half it. Take away the number you started with. Your answer is 3.

Answer 1

$(x\cdot2+6)\div2-x=3\\ (2x+6)\div 2-x=3\\ x+3-x=3\\ x-x=3-3\\ 0=0$

That number can be any real number.