Arunner completed 26.2-mile marathon in 210 minutes.

estimate the unit rate, in miles per minute.

/min

There are numerous information's of immense importance already given in the question. Based on those information's the answer to the question can be easily deduced.
The distance covered by the runner in 210 minutes = 26.2 miles
Then
The distance covered by the runner in 1 minute = (26.2/210) miles/minute
                                                                             = 0.124 miles/minute
So the estimated unit rate in miles per minute is 0.124 miles per minute.I hope the answer is correct and the procedure is clear enough for you to understand.