PLEASE SHOW HOW IT'S DONE IN EXCEL.

$1.($Lower bound of call$)$ You want to buy one American call option contract on ABC Computer, expiring in $6$ months, with a strike price of $$23.$ The current stock price is at $$24\mathrm{.}80.$

a$.$ Can the option price be lower than $$0\mathrm{.}30?$ Assume that the interest rate is $5\%.$

b$.$ Assume that you can buy the call for $$0\mathrm{.}30($which is less than the theoretical minimum$).$ How can you exploit the mispricing to make a riskless gain?