I gave this a long and hard thought because Paul Wilmott is a respected quant and I don't want to criticize his book, but am I correct in concluding that this section contains lots of errors? These are my observations and the specific section is at the bottom. I'm not trying to bash him, I'm just genuinely interested to master quantitative finance but couldn't get past this section because I think the calculations are wrong.
I finally found my error,thanks dm63 for the explanation. I had a hard time imagining the negative position value and that it implies that I also get the interest from getting cash for the short..I used a slightly different approach but got the same result.
FV when stock goes up = FV when stock goes down
Thanks for all the help guys! Really appreciate it.