Question

An element has a half-life 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 half-life 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

Answer 1

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