20c0(0.5)^0(0.5)^20
20c0(0.5)^0(0.5)^20=1*1*0.5^20
$${{\mathtt{0.5}}}^{{\mathtt{20}}} = {\mathtt{0.000\: \!000\: \!953\: \!674\: \!316\: \!4}}$$ or
$${{\mathtt{2}}}^{{\mathtt{20}}} = {\mathtt{1\,048\,576}}$$
$$\frac{1}{1048576}$$