If a original sample of radioactive uranium-238 had a mass of 400 grams, what is the total amount of the uranium-238 sample that would be left after 9 billion years?

Since the half-life is 4.5 billion years, two half-lives would have elapsed after 9 billion years. 
If you start with 400 g, half decays during the first half-life, leaving 200 g. 
During the second half-life, half of the 200 g decays, leaving 100 g.

2positives of biotechnology: 1. plants have become more resistant to viruses and other communicable pests.2. with the of biotechnology, we have been able to produce better tasting fruits as well as vegetables.2 negatives of biotechnology: 1. biotechnology poses a number of environmental threats. genetically modifies crops often infect monarch butterflies and other insect species.2. viruses could combine with crops.