The fearsome-looking (according to the fossils, of course) saber-tooth tiger has been extinct for quite a long time. A piece of 18.0-mg bone of this creature was found to contain 2.0 mg C-14. How old is this piece of bone? (The t_1/2 of C-14 is 5730 years.)

First, you need to read this explanation, and work on the problem there.

Now, it should be easy!

The amount of C-14 in % is 2.0/18.0 = 0.11 (= 11%)
Since log(M_n/ M₀) = (age/t_1/2) ´ log0.5
Thus, "age" = log(0.11)/log0.5 ´ t_1/2 = 18,250 years

<< Read a "bogus news" about saber-tooth tiger here, http://www.acclaimedmedia.com/voafa/bnn/825d.htm and relax a minute from your hard study!>>

Here is a challenging one:
The radio-active ²³⁸₉₂U decays all the way to the stable nuclide ²⁰⁶₈₂Pb after quite a few alpha and beta decays. Knowing that alpha decay emits ⁴₂He, and beta decay emits ⁰_–1e, you can find out the number of alpha and beta decays for ²³⁸₉₂U. What are they? (Answer here!)