A Table of Values of an Increasing Function $f ( x )$

A table of values of an increasing function $f ( x )$ is shown. Use the table to find an upper estimate of:
$\begin{array}{|c|c|}\hline x & f(x) \\\hline 0 & -46 \\\hline 5 & -30 \\\hline 10 & -20 \\\hline 15 & 4 \\\hline 20 & 15 \\\hline 25 & 20 \\\hline\end{array}$