Question

I have no idea how to solve these.

Answer 1

Half life is the period of time it takes for one-half of the atoms of an original isotope to decay.

$formula=(\frac{1}{2})^{n}$

In this example we have:  $(\frac{1.25 kg}{10kg})^{n}$ where n= the number of half lives.

Thus: $\frac{1.25kg}{10kg} =\frac{1}{8}$

$\frac{1}{8} =(\frac{1}{2})^{n}$ From this equation n=3. The answer to your question is: 3 half lives have passed which gives the meteorite an age of 4.5*3= 13.5 billion years.