Solution
= Solution
Condition on the intermediate parent's height and use the <Markov property>. The three mutually exclusive paths through Tall, Medium and Short contribute
$$
\begin{aligned}
\Pr(\text{short grandchild}\mid\text{tall grandparent})
&=0.6\cdot0.2+0.2\cdot0.1+0.2\cdot0.6\\
&=0.12+0.02+0.12.
\end{aligned}
$$
Equivalently, this is entry $(3,1)$ of the <matrix power> $P^2$. Hence
$$
\boxed{\Pr=0.26=26\%.}
$$