college algebra
posted by Dee on .
An isotope of lead, 201Pb, has a halflife of 8.4 hours. How many hours ago was there 40% more of the substance? (Round your answer to one decimal place.)
____hr

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

8.4