express distance as a function of time traveled.

Question

jason leaves detroit @ 2pm & drives at constant speed west. he passes ann arbor, 40 mi. from detroit at 2:50 pm.

i understand that d=rt. but i'm stuck.
is this the answer-- d(t)=t/rt ??

You could make a table like this
2pm 0m Detroit
2:50 40m Ann Arbor
or you could determine an average rate as
40mi/50min = 4/5 mi/min * 60min/hr = 48mi/hr
I'm not entirely sure what the question is asking for either, so re-read to see if there's any clue somewhere.