Santo Company stock currently trades for $50 per share. The current riskless return rate is 0.06. Under the Black-Scholes framework, what would be the standard deviations implied by six-month (0.5 year) European calls with current market values based on each of the following striking prices?

a. X = 40; c₀ = 11.50
b. X = 45; c₀ = 8.25
c. X = 50; c₀ = 4.75
d. X = 55; c₀ = 2.50
e. X = 60; c₀ = 1.25