I just need help with C and E$.$ The rest are correct.

RentAPhone is a new service company that provides European mobile phones to American visitors to Europe. The company currently

has $90$ phones available at Charles de Gaulle Airport in Paris. There are, on average, $25$ customers per day requesting a phone. These

requests arrive uniformly throughout the $24$ hours the store is open. $($Note:This means customers arrive at a faster rate than $1$ customer

per hour.$)$ The corresponding coefficient of variation is $1.$

Customers keep their phones on average $84$ hours. The standard deviation of this time is $72$ hours.

Given that RentAPhone currently does not have a competitor in France providing equally good service, customers are willing to wait

for the telephones. Yet, during the waiting period, customers are provided a free calling card. Based on prior experience, RentAPhone

found that the company incurred a cost of $$\mathrm{.}80$ per hour per waiting customer, independent of day or night.

$($a$)$ What is the average number of telephones the company has in its store? $($Round your answer to nearest whole number.$)$

The average number of telephones

$($b$)$ How long does a customer, on average, have to wait for the phone? $($Do not round intermediate calculations. Round your answer

to two decimal places.$)$

The average waiting time is

$20\mathrm{.}50$ hours.

$($c$)$ What are the total monthly $(30$ days$)$ expenses for telephone cards? $($Do not round intermediate calculations. Round your answer

to two decimal places.$)$

The total monthly expenses are

$($d$)$ Assume RentAPhone could buy additional phones at $$900$ per unit. Is it worth it to buy one additional phone?

$($e$)$ How would waiting time change if the company decides to limit all rentals to exactly $84$ hours? Assume that if such a restriction is

imposed, the number of customers requesting a phone would be reduced to $15$ customers per day. $($Do not round intermediate

calculations. Round your answer to five decimal places.$)$

The waiting time is

hours