A stock price is currently $50. It is known that at the end of two months it will be either $53 or $48. The risk-free interest rate is 10% per annum with continuous compounding. What is the value of a two-month European call option with a strikeprice of $49? Use no-arbitrage arguments.

At the end of two months the value of the option will be either $4 (if the stock price is $53) or $0 (if the stock price is $48). Consider a portfolio consisting of:

+ : shares

-1 : option

The value of the portfolio is either 48 or 53 - 4 in two months. If

48=53-4

i.e., =0.8

the value of the portfolio is certain to be 38.4.

I want to ask why 0.8 is related with 38.4? Thank you