Solution: The modal group is whichever group has the most things – in this case sunflowers – in it. Looking at the table, we see that $170 < w \leq 180$ is the modal group.
Up to 170cm, there are $5+9=14$ sunflowers. Up to 180cm, there are $14+15=29$ sunflowers. That means that the $170 < w \leq 180$ category contains the 15th to the 29th sunflower, when put in order, meaning the 20th in somewhere in there.
Therefore, $170 < w \leq 180$ group contains the median.