We need to divide the number of inches by the number of hours.
Let's first convert the two numbers into fractions.
12 1/2 = 25/2
25 1/2 = 51/2
(25/2) / (51/2) = 25/2 * 2/51 = 50/102 = 25/51 = 0.49
The average rainfall was approximately 0.5 inches per hour.