This is a solution of the heat equation
$$T=ft+f\Big (\frac{x^2-1}{2}-\frac{2}{\pi^2}\sum_{n=1}^{\infty}\frac{(-1)^n}{n^2}\exp(-\kappa n^2 \pi^2 t)\operatorname {cos}(n\pi x)\Big )$$
$T$ is temperature, $f$ a constant flux, $x$ distance, $\kappa$ a constant, $t$ time.
How to rewrite the equation to represent the changes of $t$ as a function of $f$ at a constant $T$?
Physical Description: At what $t$, $T$ reaches the given value for various $f$.