Solution by Frank Howland... Draw a figure with Person A's arrival time on x-axis, B'a arrival time on y-axis. Sketch in area where they overlap. This is an hour-long swatch going up at a slope of +1 between 1:30 and 4:30. It has to rhombus type shapes between 1 and 1:30 and 4:30 and 5. The total area of the overlap is 3 and 3/4. Area of the square is 16. Probability of overlap = 15/64.