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March 4, 2015

March 4, 2015

Posted by **Lena** on Saturday, February 19, 2011 at 2:12pm.

40K decays to 40Ar.

Half life of 40K is 1.25 billion years.

40K = 20 grams

40Ar= 5 grams

I think the formula is

Final Amount of 40K = Initial Amount of 40K * (1/2)^(time/half-life)

If I plug in the values I still don't have a value for time. And where would the grams of 40Ar fit in?

- Math/Radioactive Dating -
**bobpursley**, Saturday, February 19, 2011 at 2:31pmYou started with 25 grams, you now have 20 grams

20=25(1/2)^(t/th)

.8=(1/2)^halflives

log both sides...

log (.8)=halflives*log.5

halflivees= log(.8)/log(.5)

time= 1.25billion* log(.8)/log(.5)

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