Concept:
For grouped data,
$$
\text{Mode}
=
L+\left(\frac{f_1-f_0}{2f_1-f_0-f_2}\right)h
$$
Step 1: Identify the modal class.
The highest frequency is $12$, corresponding to the class $65-70$.
Thus,
$$
L=65,\quad f_1=12,\quad f_0=9,\quad f_2=3,\quad h=5
$$
Step 2: Substitute into the formula.
$$
\text{Mode}
=
65+\left(\frac{12-9}{24-9-3}\right)\times 5
$$
$$
=
65+\left(\frac{3}{12}\right)\times 5
$$
$$
=
65+\frac{5}{4}
$$
$$
=
66.25
$$
\[
\boxed{66.25}
\]