Concept:
For grouped data,
$$
\text{Mode}
=
L+\frac{f_1-f_0}{2f_1-f_0-f_2}\times h
$$
where
• $L$ = lower limit of modal class
• $f_1$ = frequency of modal class
• $f_0$ = frequency of preceding class
• $f_2$ = frequency of succeeding class
• $h$ = class width
Step 1: Identify the modal class.
Since the mode is $26$, the modal class is $20-30$.
Thus,
$$
L=20,\quad f_1=9,\quad f_0=k,\quad f_2=7,\quad h=10
$$
Step 2: Apply the mode formula.
$$
26
=
20+\frac{9-k}{2(9)-k-7}\times10
$$
$$
6
=
\frac{10(9-k)}{11-k}
$$
Step 3: Solve for $k$.
$$
6(11-k)=10(9-k)
$$
$$
66-6k=90-10k
$$
$$
4k=24
$$
$$
k=6
$$
Therefore,
$$
\boxed{k=6}
$$