Monthly sales of a particular personal computer are expected to decline at the following rate of $S^{\text{'}}(t)$ computers per month, where $t$ is time in months and $S(t)$ is the number of computers sold each month.

$S^{\text{'}}(t)=-20t^{\frac{2}{3}}$

The company plans to stop manufacturing this computer when monthly sales reach $700$ computers. If monthly sales now