Curtis traveled a distance of 12 miles in 2 hours from the park to his home. What was his average speed in miles per hour?

12/2 = 6 miles/hour

* (miles/hour) & (miles per hour) mean the same thing, by the way

Put the 12 miles in the numerator and the 2 hours in the denominator, divide the 2 hours by 2 (which will be 1) since we want to know the distance travelled in "one" hour. Then divide the 12 by 2 because what you do with the denominator must also be done with the numerator and vice versa.