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Radioactive decay

A threory of nuclear astrophysics is that all the heavy elements like uranium are formed in supernova explosions of massive stars, which immediately release the elements into space. If we assume that at the time of the explosion there were equal amounts of 235U and 238U, how long ago were the elements that formed our Earth released, given that the present 235U/238U ratio is 0.007? (The half-lives of 235U and 238U are 0.70 x 10^9 yr and 4.47 x 10^9 yr, respectively)

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If N0 is the initial quantity, after a time t years, the amount of quantity of radioactive material left N = N0*exp(-lambda*t), where lambda = 0.693/half life.
<br><br>Given that N0 was same initially for ...