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If the half life of an element is 5 days and 100 milligrams of the element is initially available how many milligrams (mg) of the element is available after 20 days?
Wiki User
∙ 7y ago
The answer is 2.
If after one half-life only half of the element remains, then after another half-life half of what was there only remains. So a half of a half is a quarter (or a fourth). So that's 2 half-lives.
Wiki User
∙ 12y ago
Wiki User
∙ 7y agoAfter 5 days, half the element is gone - therefore after..
5 days = 50mg...
10 days = 25mg...
15 days = 12.5mg...
20 days = 5.25mg...
and... after 25 days = 3.125mg.
Wiki User
∙ 7y agoDivide the total time by the time of the half life. In your case, 20 days divvied by a half life of 5 days gives you 4 periods. Each period will lose half of what is there. At the end of the 1st 5 day period, you will have gone from 100 mg to 50. 2nd 5 day period, drops from 50 to 25 (half of what was left) and the end of the 3rd 5 day period, 12.5 (half of 25) At the end of your last 5 day period, you will have half of 12.5 mg.
Wiki User
∙ 12y ago3 half-lives
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