# Boron has two isotopes $_5B^{10}$ and $_5B^{11}$. If atomic weight of Boron is 100.81 then ratio of $_5B^{10}$ to $_5B^{11}$ in nature will be :
$\begin{array}{1 1} (1)\; 15 : 16 \\ (2)\; 19 : 81 \\ (3)\; 81 : 19 \\ (4)\; 20 : 53 \end{array}$