John wants to get to the bus stop, the bus stop is across a grassy park, 2000 feet west and 600 feet north of his starting position. John can walk west along the edge of the park on the sidewalk at a speed of 6 ft/sec and 4 ft/sec through the grass. How far should he walk on the sidewalk before veering off onto the grass if he wishes to get to the bus stop in exactly 7 min 30 sec? If there is more than one valid answer, list all valid answers as a comma-separated list.

Answer: