Answer
3100 atoms
Work Step by Step
Number of half-lives= $\frac{t}{t_{1/2}}=\frac{10\,d}{2.0\,d}=5.0$
As the sample undergoes 5 half-lives, the initial number of atoms decreases by half 5 times.
Number of atoms left=$100,000\times(\frac{1}{2})^{5}=\frac{120}{16}=3.1\times10^{3}\,atoms$
