The half-life of a radioactive substance is 400 years. If you start with some amount of this substance, what fraction will remain in 120 years?

Adolphin is swimming with her friend. the dolphin jumps to a height of 4.54.54, point, 5 meters above the surface of the water as her friend swims 9.89.89, point, 8 meters directly below her. what is the position of the dolphin's friend relative to the surface of the water? meters