Answer:
After 4 minutes
Explanation:
After 1 min: Ant 1 is 9.75 mm underground. Ant 2 is 14.5 mm underground.
After 2 min: Ant 1 is 9.5 mm under. Ant 2 is at 14 mm under.
After 3 min: Ant 1 is 9.25 mm under.
After 4 min: Ant 1 is 9 mm under. Ant 2 is 13.5 mm under.
Ant 2 is now closer to the surface than Ant 1.