15/6 miles in 1/4 hour. what is mph?

Most of the information's that are required for solving the question is already given in the question. It is important to look at those information's very minutely.
The thing that is already given is
In 1/4 hours the distance traveled is = 1 5/6 miles
                                                          = 11/6 miles
Then
In 1 hour the distance traveled = (11/6)/(1/4) miles
                                                  = (11 * 4)/6 miles
                                                  = 44/6 miles
                                                  = 22/3 miles
                                                  = 7 1/2 miles
                                                  = 7.33 miles
So the answer to the question is 7 1/2 miles per hour or 7.33 miles per hour and it has been confirmed from the above deduction. I hope the procedure is simple enough for you to understand.

Documentation and time management