I have some queries with the above question.

1. why will the portfolio value be the share of the stock price minus f (option value at t=0)
2. can I use e^-rt and (1+r)^-n interchangeably for binomial trees?

The portfolio is riskless when its values are equal at time T : 221=18, giving =0.25 and PortfolioValueatT=$4.5 The present value of the portfolio is =e0.034.5=4.367 and this must be the value at time 0 . That is, Portfolio Value at time 0 =4.367=5f Therefore, f=0.633