John is planning a road trip and wants to visit two cities, City A and City B, which are located along a straight highway. City A is located at the 20-mile mark on the highway, and City B is located at the 80-mile mark. John wants to find the midpoint of the distance between these two cities. Additionally, he plans to make a stop exactly halfway between the midpoint and City A. What is the distance of the stop from the starting point of the highway? Let's think step by step and output the final answer within \boxed{}.
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