Question

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Answer 1

More than 75 percent

Explanation:

Let us carefully analyze this problem:

The half life of the rubidium isotope = 50 billion years

   The half-life of a radioactive isotope is the time it takes for it to decay to half of its original amount.

  Every 50years, we know that a rubidium isotope would split into half.

Initially when the isotope was formed, we would only have the parent atoms there. There wont be any daughter;

  ratio of parent to daughter = 100 : 0

When the isotope decays to half; we would have equal number of parent and daughter:

  ratio of parent to daughter at half life = 50 : 50

The age of the earth is about 4.6billion years;

this is;

          $\frac{4.6}{50}$ x 100 = 9.2% of the half-life of the rubidium isotope.

At this time, we should know that only 9.2 percent of the daughter would have been formed.

The remaining parent would be 100 - 9.2 = 90.8% would still be left undecayed.

The ratio of parent to daughter  = 90.8 : 9.2

This is more than 75% of the parent

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