The half-lives of 235U and 238U are 7.04 × 108 years and 4.47 × 109 years, respectively, and the present abundance ratio is 238U>235U 5 137.7. It is thought that their abundance ratio was 1 at some time before our earth and solar system were formed about 4.5 × 109 years ago. Estimate how long ago the supernova occurred that supposedly produced all the uranium isotopes in equal abundance, including the two longest lived isotopes, 238U and 235U.

235U decays to 207Pb via the decay series as shown. The first decay in the chain, that of 235U, has a halflife of 7.0 × 108 years. The subsequent decays are much more rapid, so we can take this as the half-life for the complete decay of 235U to 207Pb. Certain minerals exclude lead but not uranium from their crystal structure, so when the minerals form they have no lead, only uranium. As time goes on, the uranium decays to lead, so measuring the ratio of lead atoms to uranium atoms allows investigators to determine the ages of the minerals. If a sample of a mineral contains 3 atoms of 207Pb for every 1 atom of 235U, how many years ago was it formed?

Could i have some help breaking down a,b, and c?

In heavy rush-hour traffic, you drive in a straight line at 12 m/s for 1.5 minutes, then you have to stop for 3.5 minutes, and finally, you drive at 15 m/s for another 2.5 minutes. Plot a position-versus-time graph for this motion. Your plot should extend from t = 0 minutes to t = 7.5 minutes. Assume x=0 and t=0 at the start of your motion.