Answer
$384$.
Work Step by Step
The given expression is
$=16\cdot 24$
$16$ is $4$ less than $20$, while $24$ is $4$ more than $20$.
Write as product of difference and sum.
$=(20-4)(20+4)$
Use sum and difference pattern $(a-b)(a+b)=a^2-b^2$.
We have $a=20$ and $b=4$.
$=20^2-4^2$
Simplify.
$=400-16$
Hence, the product is $384$.