Suppose that a share of Shell costs $21 now and that the continuously compounded risk-free rate is 1%. Compute the value...

Suppose that a share of Shell costs $21 now and that the continuously compounded

risk-free rate is 1%. Compute the value of a put option to sell Shell for $22 half a

year from now, under the assumption that in half a year, the share costs either $20

or $23.