Posted by **Dee** on Sunday, November 17, 2013 at 4:10pm.

An isotope of lead, 201Pb, has a half-life of 8.4 hours. How many hours ago was there 40% more of the substance? (Round your answer to one decimal place.)

____hr

- college algebra -
**Steve**, Sunday, November 17, 2013 at 4:14pm
If there was 40% more, then that's 1.4 times what's there now.

So, starting with an original amount of 1, there is now 1/1.4 of it left.

(1/2)^(t/8.4) = 1/1.4

Now just solve for t hours.

Note: It will be less than 8.4, since 8.4 hours ago, there was 100% more.

- college algebra -
**Dee**, Monday, November 18, 2013 at 9:24pm
not sure how to find the t, tried using the log approach but can't get the right answer.

- college algebra -
**Anonymous**, Wednesday, July 27, 2016 at 8:50pm
8.4

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