SOLUTION: Please help me with this word problem, I'm out of my depth: If 12 g of a radioactive substance are present initially and 4 yr later only 6 g remain, how much of the substance will

Question 703263: Please help me with this word problem, I'm out of my depth:
If 12 g of a radioactive substance are present initially and 4 yr later only 6 g remain, how much of the substance will be present after 7 yr?
Thank you,
Samara
Answer by josgarithmetic(39625) (Show Source): You can put this solution on YOUR website!
Radioactive decay may work like an exponential decay.
Using t for years, A[0] for starting amount, A[t] for amount at time t,
We may use formula,

It's not clear if you mean first 4 years and then 3 more years, or are you just saying, "half life is 4 years"+". After 7 years."

Starting with half life, we want to find k.
In a 4 year period, t=4, and A quantity will be reduced to (1/2)*A.

Computing k gives us k = 0.173.
Our decay formula then is

NOW, if you want the quantity remaining after 7 years and the starting quantity is 12 grams, then:

A[7]=3.6 grams

RELATED QUESTIONS
My problem is with entering e raised to a negative power on my TI-84 Plus calculator. I... (answered by Edwin McCravy)

If 10 g of a radioactive substance are present initially and 9 yr later only 5 g... (answered by josgarithmetic)

If 10g of a radioactive substance are present initially and 9yr later only 5 remain, how... (answered by ikleyn)

If 800g of a radioactive substance are present initially and 8 years later only 450g... (answered by greenestamps,ikleyn)

If 18g of a radioactive substance are present initially and 5 yr later only 9 g remain,... (answered by josgarithmetic)

If 10g of a radioactive substance are present initially and 3 years later only 5g remain, (answered by CubeyThePenguin)

Here i my word problem......please help! the amout of radioactive substance present,... (answered by stanbon)

Please help me solve this word problem: A certain radioactive element decays at a rate of (answered by stanbon)

Please help me solve this word problem: A certain radioactive element decays at a rate of (answered by Edwin McCravy)