SOLUTION: If 18g of a radioactive substance are present initially and 5 yr later only 9 g remain, how much of the substance will be present after 11yr ? after 11yr there will be ___g of a r

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Question 928743: If 18g of a radioactive substance are present initially and 5 yr later only 9 g remain, how much of the substance will be present after 11yr ?
after 11yr there will be ___g of a radioactive substance.
round the final answer to the nearest thousandth as needed. round all intermediate values to the nearest thousandth as needed.
Answer by josgarithmetic(39625) About Me  (Show Source):
You can put this solution on YOUR website!
Decay equation p=18%2Ae%5E%28-kt%29 and you first want to take the given data and use to find k. t is time in years, p is quantity remaining after time t.

ln%28p%29=ln%2818%29-kt
ln%28p%29-ln%2818%29=-kt
-k=%28ln%28p%29-ln%2818%29%29%2Ft
k=%28ln%2818%29-ln%28p%29%29%2Ft
Substitute the given data point:
highlight_green%28k=%281%2F5%29ln%282%29%29, and you can compute this value yourself of k.


How much is p at t=11?
You know k now, and given t.
Evaluate p.