Recall the relationship between the present amount of element and the initial amount of element in terms of the decay constant, $N=N_0{e}^{-\lambda t}$, now the relation between decay constant and half-life, $t_{\frac{{}_1}{2}}=\frac{0.693}{\lambda }$, $\Rightarrow N=N_0{e}^{-\frac{0.693t}{t_{{\displaystyle \frac{1}{2}}}}}$

$$$\Rightarrow N={10}^{10}{\mathrm{e}}^{-0.35}=7\times {10}^9$.