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I have been set this question as an assignment and I spent about an hour researching it yesterday and came up with what I thought was the right answer, but it has turned out to be wrong.

"A theory of astrophysics proposes that all the elements heavier than iron are formed in supernova explosions ending the lives of massive stars. If we assume that at the time of the explosion the amounts of 235U and 238U were equal, how long ago did the star(s) explode that released the elements that formed our Earth? The present 235U / 238U ratio is 0.00725. The half-lives of 235U and 238U are 0.704 x 10^9 years and 4.47 x 10^9 years."

Here is what I did.

Reciprocal of 0.00725 = 137.93

Used log to find that 2^7.1078 = 137.93

Multiplied 7.1078 by 0.704 x 10^9 to get 5,003,891,200 years.

But this answer turned out to be wrong. Can anyone guide me in the right direction?

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# Homework Help: Tricky radiation half-life question

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