Answer
$3\times 10^{19}~atoms$
Work Step by Step
If the half-life is $6.5~hours$, then $26~hours$ is 4 half-lives.
The fraction of a sample that remains at the end of 4 half-lives is $(\frac{1}{2})^4 = \frac{1}{16}$
We can find the number of atoms that remain:
$(\frac{1}{16})(48\times 10^{19}~atoms) = 3\times 10^{19}~atoms$
