Answer
$1540a^3b^{19}$
Work Step by Step
The $k+1$th term of the expansion of $(a+b)^n$ is:
$$T_{k+1}=\binom{n}{k}a^{n-k}b^k,\text{ where }k=0,1,2,\dots,n.\tag1$$
We find the $20$th term of the expansion of $(a+b)^{22}$ by substituting $k=19$, $n=22$ in Eq.$(1)$:
$$T_{20}=\binom{22}{19}a^{22-19}b^{19}=1540a^3b^{19}.$$
Therefore the term on the $20$th position is $1540a^3b^{19}$.
