1. John said to Fred, "If I can guess the number you are left with will you give me the amount in money?"
  2.     "Yes," said Fred. "I'm not averse to this. So long as I can ask you."
  3.     So John said to Fred, "Think of a number and don't tell me."
  4.     So he did and he didn't.
  5.     "Add three," said John. "Multiply by two. Subtract by four. Alright?"
  6.     "Yes. Got it."
  7.     "Divide by two and subtract the original number."
  8.     "Right," said Fred.
  9.     "So you owe me one pound," said John.     "Because you have been left with a one."
  10.     "Very good indeed," said Fred. "So the deal is I can now ask you."
  11.     "Yes," said John, "So pay up."
  12.     He did, and said, "So you think of a number."
  13.     "Yes."
  14.     "Now add to it the next number up; that is, one greatest."
  15.     "Got it."
  16.     "Add nine. Then divide what you have got by two. In other words, halve it."
  17.     "Half the value it is."
  18.     "And then subtract from that the number you started with."
  19.     "I have."
  20.     "Give me five pounds."
  21.     "Fantastic. I am much the poorer."
  22.     "Yes," said Fred, "In algebra it is called an identity, because it is not even an equation. No matter what is the starting point, it always comes back to the figure I selected and not the one you select. In other words x is altered but y stays the same. In an equation the attempt is to find out how another variable y is affected by the number chosen x.
  23.     "It's quite a mute point. Which means you could guess the number I start with?"
  24.     "Yes, if you tell me the final answer; anyone can adopt the basic approach to also identify the first number.
  25. So it is like the card trick, like the one broadcasted the other day on the magic show.
  26. So think of a number."
  27.     "Right," said John.
  28.     "Double it. Add four."
  29.     "With you."
  30.     "Now divide that by two and then add thirteen."
  31.     "So now you need to ask me what I have got left."
  32.     "What have you got left?"
  33.     "Twenty one."
  34.     "You started with six. You owe me six pounds."
  35.     "Hang on a minute; this was not for money."
  36.     "Yes but the rules are pretty much the same. In this case they were achieved by doing two x plus four in brackets then divided by two and adding thirteen."