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Answer 1 · 2017-05-29T10:55:55

Pu-239 has a half life of 24100 years.

Decay reaction:

$C=C_0*e^-(r*t)$

First let me compute r (decay coefficient) for Pu-239.

$5=10*e^-(r*24100)$

$ln 0.5 = -24100*r$

$r = 0.00002876$ 1/year

Now you can compute what amount of Pu-239 will be after 6500 years.

$C = 10*e^-(0.00002876*6500)$

$C=10*0.8295$

$C=8.295$ grams of Pu-239.

Your answer is 8.295 grams.

Reference:

Half-life of Plutonium 239. Retrieved on the internet May 29, 2017. hypertextbook(dot)com(slash)facts(slash)1999(slash)JaniceChing(dot)shtml