Explain why it takes longer to hard boil an egg at high altitudes than at sea level?

It takes longer to hard boil and egg at high altitudes than at sea level because aht high altitude the boilling temperature or water is lower than at sea level.

Explanation:

The boling point of water at sea level is 100°C, as the altitude increases, the atmospheric pressure descends, which means the the water starts boiling at lower temperature.

Once, the water starts boiling its temperature remains constant: the new heat added from the stove is used to evaporate the water while its temperature remains unchanged.

So, at high altitudes the egg will be cooked in water at lower temperature, which means that it will be cooked more slowly and so it will take longer.

For example, at 3000 m the boiling point of water is about 90°C, while (as already said) the boiling point of water at sea level is 100°C.