A question about percents This is the body in the question about percents.
$8,000$ is $40\%$ of the whole

Divide by $4$

$2,000$ is $10\%$ of the whole

Muliptly by $10$

$20,000$ is $100\%$ of the whole. 

This answer doesn't really depend on algebra (but essentially the same) ,  using just your head.  But, like other answers suggest, mastering algebra would help you better to solve another problem.
If:
$$
\begin{split}
    40\% &amp;= 0.4\\
    8000 &amp;\to 0.4\\
    x &amp;\to 1
\end{split}
$$
then:
$$
   x = \frac{8000 \cdot 1 }{0.4} = \frac{8000}{0.4} = 20000
$$