PLEASE SHOW HOW IT'S DONE ON EXCEL. PLEASE DON'T USE AI TO ANSWER. I WILL RATE.

$($Finding rf Stockprice today Putprice $)$ In a binomial model a European put option is written on a stock

selling today for $$30.$ The exercise price of the put option is $$40.$ The put option$$s payoffs are $$20$ and $$5.$ The price of the put is $$9\mathrm{.}50.$ What is the riskless interest rate? Assume that the basic period is $1$ year.