Ask Marilyn ® by Marilyn vos Savant is a column in Parade Magazine, published by PARADE, 711 Third Avenue, New York, NY 10017, USA. According to Parade, Marilyn vos Savant is listed in the "Guinness Book of World Records Hall of Fame" for "Highest IQ."
In her Parade Magazine columns of July 18, 1993 and July 25, 1993, Marilyn presented a simple algebra word problem as a Brainteaser. However, she interpreted the word "instead" to mean "and." As a result, she came up with the wrong answer.
This is a simple algebra word problem, and the important word here is instead. Your friend intended to give back your porcupine, but instead gave you one of his. But he only gave you one porcupine, not two!
There were six porcupines to begin with. You had four, and your friend had two. When you gave him one, you both had three. When he gave you one of his, it left him with two (the same number he originally started with) and you with four.
This is really a dispute over grammar, not arithmetic. The "instead" in question can be interpreted in two different ways:
I believe that math problems should be written clearly and simply. The challenge of interpreting a narrative is a literary concern, not a mathematical one. The real mistake with this problem is the writer's ambiguous wording.
Thank you for suggesting this alternative interpretation. I still believe that my interpretation is a better one, since it seems more intuitive for "instead" to refer to the closest previous antecedant (giving it back, rather than accepting your gift). I agree, however, that the real mistake is Marilyn's ambiguous wording.
There are originally EIGHT porcupines: you have 5, your friend has 3. You give your friend one, and now you both have 4. But the one of HIS that he returns is his pregnant female, and when she gives birth you have 6, he has 3, and there are a total of NINE.