Answer
$1540a^{3}b^{19}$
Work Step by Step
Step 1. With the Binomial Theorem, the general form of the $r$th term of the expansion of $(a+b)^n$ is given by:
$\begin{pmatrix} n\\r-1 \end{pmatrix}a^{n-r+1}b^{r-1}$
Step 2. Wtih $n=22, r=20$, we have the $20$th term as $\begin{pmatrix} 22\\19 \end{pmatrix}a^{3}b^{19}=\frac{22!}{19!3!}a^{3}b^{19}=1540a^{3}b^{19}$
