Runner D runs 3 miles in 2/3 hours. To find their speed, we can divide the distance by the time. So, the answer would be 3 miles divided by 2/3 hours. Let's simplify that. 2/3 hours is the same as 2 divided by 3, which is approximately 0.67. Now, we can calculate the speed by dividing 3 miles by 0.67 hours. The answer is approximately 4.48 miles per hour.