Science
posted by Lynette .
An element has a halflife of 30 years. If 1.0 mg of this element decays over a period of 90 years, how many mg of this element would remain?
Begin amount is 1.0
elapsed time is 90y half life 30 years
n=9/30 n=3
90/2^2
90/8 = 11.25mg
thinking it's wrong not sure what I missed.
A 2.5 gram sample of a radioactive element was formed in a 1960 explosion of an atomic bomb at Johnson Island in the Pacific Test Site. The halflife of the radioactive element is 28 years. How much of this element will remain after 112 years?
I tried it this way 112/28= 4 half lives then 2.5/3 1.25
1.25/2=0.625
06.25/2= 0.3125
0.3125/2=0.15625 gram
which is the correct way to do these problems. Am I doing that the correct way

Science 
Chris
1. 30 years= 3 1/2 lives so
1.0mg >30yrs> 0.5mg
0.5mg>30yrs>0.25mg
0.25mg>30yrs>0.125mg
2. correct
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An element has a halflife of 30 years. If 1.0 mg of this element decays over a period of 90 years, how many mg of this element would remain?