On Sat, Jun 5, 2021, 1:58 PM Karl <gmkarl@gmail.com> wrote:

X(t) ~= (number of people who have arrived between 0 and t) - (number of people who have been fully served between 0 and t)

Both of these values are calculable.

S_n is the number of minutes that pass after person (n-1) arrives, before person n arrives.