Response to the following problem:
On Saturdays customers arrive at Patterson's Custom Jewels according to a Poisson distribution at a mean rate of 7 per hour. The store has 4 clerks working on Saturday, and the time a clerk spends with a customer follows an exponential distribution with a mean of 20 minutes. For security reasons, the entrance to the store is locked, and a maximum of 8 customers can be present in the store at any one time. It is assumed that customers who are not granted admittance to the store will leave and shop elsewhere.
Determine the percentage of customers who will not be granted admittance to the store.