Patients arrive at a doctor's office on average every $15$ minutes. They first see a physician's assistant for a preliminary review of their symptoms, which takes on average $5$ minutes. What is their arrival rate?

Question $4$ options:

a$)$

$2\mathrm{.}5/$hr$.$

b$)$

$4/$hr$.$

c$)$

$0\mathrm{.}25/$min

d$)$

$1\mathrm{.}5/$min

e$)$

none of these