Answer
$\boxed{0.0001811981201171875}$
Work Step by Step
The general formula for the distribution of the heads and tails should be found.
The chance of having any specific side of the coin is $1/2$ and the way to choose n heads is $_{20}C_n$.
$$_{20}C_2.*(1/2)^{20}=\boxed{0.0001811981201171875}$$
