@changasnr,
Divide 1 2/3 by 50 and multiply the result by 60.
1 2/3 = 5/3 so you are going to divide 5/3 by 50/1
That (after the fraction is simplified) is 1/30 so Mara's speed is 1/30th of a mile per minute.
Now you are going to multiply 1/30 by 60 (or 60/1) and this will give you 60/30 or in other words 2.
Mara goes 2 miles per hour.