college algebra
posted by Dee .
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
Respond to this Question
Similar Questions

linear/exponential
This question involves the halflife formula. In this exercise, we are to give a halflife for an exponentially decaying quantity. Need answer to the following: The halflife of a drug in the bloodstream is 4 hours. By what factor … 
chm
An unknown radioactive substance has a halflife of 3.20 . If 34.8 of the substance is currently present, what mass was present 8.00 hours ago? 
College Algebra
Use a calculator to help solve the problem. One isotope of holmium, 162Ho, has a half life of 22 minutes. The halflife of a second isotope, 164Ho, is 37 minutes. Starting with a sample containing equal amounts, find the ratio of the … 
college algebra
In 4 years, 30% of a radioactive element decays. Find its halflife. (Round your answer to one decimal place.) _____ yr 
Halflife
After two hours, 1/16 of the initial amount of a radioactive isotope remains undecayed. What is the halflife of the isotope? 
college algebra
Use a calculator to help solve the problem. An isotope of lead, 201Pb, has a halflife of 8.4 hours. How many hours ago was there 40% more of the substance? 
college algebra
One isotope of holmium, 162Ho, has a half life of 22 minutes. The halflife of a second isotope, 164Ho, is 37 minutes. Starting with a sample containing equal amounts, find the ratio of the amounts of 162Ho to 164Ho after one hour. … 
math with application
If 12 % of a radioactive substance decays in 7 hours, what is the halflife of the substance? 
Algebra
For a nonsmoker, the biological halflife of theophylline is 8 hours. Find the amount of the drug retained in a nonsmoker's system 18 hours after taking a oneunit dose. (Round your answer to three decimal places.) 
PreCalc
The initial size of a culture of bacteria is 1500. After 1 hour the bacteria count is 12000. (a) Find a function n(t) = n0e^rt that models the population after t hours. (Round your r value to five decimal places.) n(t) = (b) Find the …