Marnie was walking 1 mile and it took her 10 minutes and 12 seconds. how would you convert that into a proportion of miles per hour?

Let us first convert 10 minutes 12 seconds in seconds.
So
10 minutes 12 seconds = (10 * 60 ) + 12 seconds
                                       = 600 + 12 seconds
                                       = 612 seconds
Now
Let us convert the 612 seconds into minutes
612 seconds = 612/60 minutes
                      = 10.20 minutes
Now
The distance covered by Marnie in 10.2 minutes = 1 mile
hen
Distance covered by Marnie in 60 minutes = (60/10.2) miles
                                                                     = 5.88 miles
Then the unit rate will be 5.88 miles per hour

Miles per hour is:            Distance in Miles / Time in hours

Distance = 1 mile

Time = 10 minutes 12 seconds

12 seconds is =    (12 / 60) minutes.          Since 60 seconds = 1 minute

                         =    (1/5) minutes

10 minutes 12 seconds =  10  +  1/5  =  51/5  minutes.

To hour:        51/5  Divided by 60.                    Since 60 minutes = 1 hour

                    =  51/5  *  1/60 =  51/300  = 17 / 100 hour      (Divided through by 3)

Miles per hour =     1 mile  /  (17/100) hour

                        =    1 * 100/17 =  5.882 miles per hour.

I hope this helps.

What's the denominator