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the rubidium isotope 87Rb is a beta emitter with a half-life of 4.9E10 y that decays into 87Sr. It is used to determine the age of rocks and fossils. Rocks containing the fossils of early animals contain a ration of 87Sr to 87Rb of 0.090. Assuming that there was no 87Sr present when the rocks were formed, calculate the age of these fossils. Answer in units of y.

Assume we start with 100 atoms Rb87 @ t=0 when the fossil just started to form. Then after some time, y will have disintegrated and formed Sr87. Thus 100-y will be the number of Rb87 atoms left.
(Sr87/Rb87)=0.090 from the problem.
Solve for y and 100-y

k = 0.693/t1/2= ??
ln(No/N) = kt
No = 100
N = 100 - y from above.
k = from above
solve for t, the age of the fossil.

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