316 atoms of C-13
There are three naturally occurring isotopes of carbon. C-12, C-13, and C-14. C-13 comprising 1.055% of the total carbon (on average).
So to get the number of C-13 atoms in a sample of 3.0000x10^4 atoms, just multiply
3.0000 x 10^4 * 0.01055 = 316 atoms on average.
