Two European call options have the same underlying asset and have premiums $0.756 and $1.720, with strike prices $25.20 and $22.00, respectively. Both calls will expire in four months. The continuously compounding interest rate is r = 2.8% pa and the underlying asset is currently trading at $21.62.

(a) Why do these two calls have different premiums?

(b) Use these two calls to calculate two different values of the implied volatility to three significant figures.