This question is asking for how much samble is left if the hlaflife is 4500 days and the sample was left for 13,500 days. The answer is 1/8, but I can't see why 3 half lifes would have passed. Wouldn't half the sample be left 4500 days, 1/4 after 9000 days, and 1/8 after 18000. Wouldn't there have to be more than 1/8 at 13500. Can someone explain where I'm going wrong?I have attached the relevant parts of the question.
AAMC Sec 1 QB #60
AAMC Sec 1 QB #60
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Re: AAMC Sec 1 QB #60
Hi Snazia,
The idea here is that halflife intervals are obtained as follows: If the halflife time is n days, the first halflife will take place in n days, the second one in 2n (or n+n days), the third one in 3n (or 2n + n) days, the fourth one in 4n (or 3n + n) days, etc. So if it was 4 days, the sequence of halflife intervals would be 4, 8, 12, 16, 20 days, etc.
Therefore, you're right that the first halflife in this case would pass in 4500 days and the second one would pass in 9000 days. Then, the third one will be at 3*4500 or 9000+4500 days (=13,500 days). It looks like the obtain 18,000, you multiplied the second halflife interval (9000 days) by 2, when instead, you need either 3x the halflife or the second halflife interval + the halflife itself.
Hope this helps clarify this question! If not, I'd suggest writing it out with some sample halflife times.
The idea here is that halflife intervals are obtained as follows: If the halflife time is n days, the first halflife will take place in n days, the second one in 2n (or n+n days), the third one in 3n (or 2n + n) days, the fourth one in 4n (or 3n + n) days, etc. So if it was 4 days, the sequence of halflife intervals would be 4, 8, 12, 16, 20 days, etc.
Therefore, you're right that the first halflife in this case would pass in 4500 days and the second one would pass in 9000 days. Then, the third one will be at 3*4500 or 9000+4500 days (=13,500 days). It looks like the obtain 18,000, you multiplied the second halflife interval (9000 days) by 2, when instead, you need either 3x the halflife or the second halflife interval + the halflife itself.
Hope this helps clarify this question! If not, I'd suggest writing it out with some sample halflife times.
Andrew D.
Content Manager, Next Step Test Prep.
Content Manager, Next Step Test Prep.
Re: AAMC Sec 1 QB #60
Ah, I see. I'm not sure why I doubled the time like that. Thank you!

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 Posts: 520
 Joined: Mon May 23, 2016 1:47 pm