Describe the lifestyles of plankton, nekton, and benthos. why is it true that plankton account for a much larger percentage of the ocean’s biomass than the benthos and nekton?

Ihave no idea how to solve these. plus there's another question that is 2) a 10 kilogram meteorite is collected in space and brought back to earth. it is composed of 1.25 kg of uranium-238 and 8.75 kg of lead-206. how many u-238 half lives have passed? what is the approximate age of the meteorite?