# college algebra

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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 -

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 -

not sure how to find the t, tried using the log approach but can't get the right answer.

• college algebra -

8.4

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