Solution
= Solution
Under fairness every expected count is $90/6=15$. Therefore
$$
\begin{aligned}
X^2
&=\frac{(20-15)^2+(15-15)^2+(12-15)^2
+(17-15)^2+(9-15)^2+(17-15)^2}{15}\\
&=\frac{25+0+9+4+36+4}{15}
=\boxed{5.2}.
\end{aligned}
$$
= Solution
Under fairness every expected count is $90/6=15$. Therefore
$$
\begin{aligned}
X^2
&=\frac{(20-15)^2+(15-15)^2+(12-15)^2
+(17-15)^2+(9-15)^2+(17-15)^2}{15}\\
&=\frac{25+0+9+4+36+4}{15}
=\boxed{5.2}.
\end{aligned}
$$