carbon dating

[Oh_Gee]

from GS-7

can someone explain the answer to me?

I thought the decay would start at 1/20 to 1/40 to 1/80 which is 3 half lives

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[Chrisz]

You sure it is 3 not 2? What is the initial amount, what is the final amount after each half life, and then tell me how many half-lives has passed?

 

[type12]

1/20 to 1/40 is one half life
1/40 to 1/80 is another half life

 

[NextStepTutor_1]

Bingo @type12.
Be careful with your analysis here. When considering half-lives focus on the progression from one amount to another, not the number of starting and ending points per se.
The progression is 1 while you move from 2 numbers and there are 2 progressions in this example - one from 1/20 to 1/40 and the other from 1/40 to 1/80. So there are 2 half lives.

 

[Oh_Gee]

oh wow thanks guys. this is the first time i've gotten one of these problems wrong. guess the different wording just messed me up