$6\mathrm{.}6(3$ pts$.)$ At a walk$-$in medical clinic, the time between customer arrivals averages $30$ seconds. On average, a customer spends $4$ minutes at the center. What is the average arrival rate per hour?

a$)15$ per hour.

b$)30$ per hour.

c$)90$ per hour.

d$)120$ per hour.

e$)$ none of the above.

$6\mathrm{.}7(3$ pts$.)$ Suppose that in a waiting line problem, the average time between customer arrivals increases. Then, the arrival rate would be expected to

a$)$ increase.

b$)$ decrease.

c$)$ stay the same.

$6\mathrm{.}8(3$ pts$.)$ At a busy coffee shop, the time between customer arrivals averages $1$ minute. On average, a customer spends $2$ minutes in service receiving their coffee order. What is the average service rate per hour?

a$)3$ per hour.

b$)30$ per hour.

c$)60$ per hour.

d$)180$ per hour

e$)$ none of the above.