35. If the half-life of a radioactive isotope is 200 years, and a specimen contained 5 grams of parent isotope and 15 grams of daughter product, then how many half-lives have elapsed.

Isotopes such as 26Al (half-life: 7.2105 years) are believed to have been present in our solar system as it Formed, but have since decayed and are now called extinct nuclides. (a) 26Al decays by + emission or electron capture. Write the equations for these two nuclear transformations. (b) The earth was formed about .4.7109 (4.7 billion) years ago. How old was the earth when 99.999999% of the 26Al originally present had decayed?