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?

Half life is the period of time it takes for one-half of the atoms of an original isotope to decay.

In this example we have:   where n= the number of half lives.

Thus:

From this equation n=3. The answer to your question is: 3 half lives have passed which gives the meteorite an age of 4.5*3= 13.5 billion years.  

stress fractures occur largely in the feet and legs and are common in sports that require repetitive impact, primarily running/jumping sports such as gymnastics or track and field.

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.