Risk free hedge

Looking at CFAI reading 53, problem question 4B, question says “calculate number of units of the underlying stock that would be needed at each point in the binomial tree to construct a risk-free hedge”. In part A) we calculate call price today = 12.85. And hedge ratio = 0.7559. But what i don’t get is how do they determine the option is over priced when there is no price given to compare with?