# Adjusting text size to fit tabu column width

I am using tabu to create math tables. One of my equations is too long and exceeding the column width of the table, like so:

How can I make LaTeX automatically adjust the font size of the equation to fit within the column width?

Here is my MWE:

\documentclass[british, 11pt, reqno]{amsart}
\usepackage[utf8]{inputenc}
\usepackage{booktabs}
\usepackage{tabu}

\begin{document}

\begin{tabu} to \textwidth{X[c] X[c] X[c] X[c] X[c]}
\toprule
Test & Test & Test & Test & Test\\
\midrule
$r_0(P_2)=\frac{1}{5}$ & $r_1(P_2)=\frac{4}{15}$ & \begin{aligned} r_2(P_2)&=\frac{r_2(P_1)}{3} + \frac{r_2(P_4)}{1}\\ &=\frac{1}{6} \end{aligned} & $r_3(P_2)=\frac{7}{45}$ & 1\\
\bottomrule
\end{tabu}

\end{document}