Sarah bikes 15 miles in 70 minutes. which is her average speed per minute?

The definition of average speed is

        (total distance covered) divided by (time to cover the distance) .

If you didn't remember that, you can still solve the problem
if you Use your units !

You want a number that has units of  [ mile / minute ].

That's what you get if you divide the miles by the minutes.

         Average speed  =  (15 miles)  /  (70 minutes)

                                     =       ( 15 / 70 )   miles/minutes

                                     =          0.214 mile / minute  (rounded) 

The answer to the question is (a)