Question $4.$

$[25]4-$a$)$ Suppose the risk$-$free rate is $5\%$ and $S(0)=21.$ Consider an American put option and

European call option, both with strike price $$20$ and expiration date in three months. The call

option's price is $$4$ and the put option's price is $$1.$ Find an arbitrage opportunity using only:

$($i$)$ underlying asset $S,($ii$)$ risk$-$free money market fund, $($iii$)$ American put option, European call

option.