6. A stock price is currently at $20 and has a volatility of 20% p.a. The risk-free rate is 4% p.a. with continuous compounding. Use a three-step binomial tree model with step size 1 year to compute the arbitrage-free price of an American put option written on that stock with strike price K $20 and maturity T = = 3 years. 6. A stock price is currently at $20 and has a volatility of 20% p.a. The risk-free rate is 4% p.a. with continuous compounding. Use a three-step binomial tree model with step size 1 year to compute the arbitrage-free price of an American put option written on that stock with strike price K $20 and maturity T = = 3 years