Concept:
For grouped data,
\[
\text{Mode}
=
l+\frac{f_1-f_0}{2f_1-f_0-f_2}\times h
\]
where
\[
l=30,\quad h=10,\quad f_1=8,\quad f_0=5,\quad f_2=x
\]
since the modal class is \(30-40\).
Step 1: Substitute the given values.
\[
33.75
=
30+\frac{8-5}{2(8)-5-x}\times10
\]
\[
33.75-30
=
\frac{3}{11-x}\times10
\]
\[
3.75=\frac{30}{11-x}
\]
Step 2: Solve for \(x\).
\[
3.75(11-x)=30
\]
\[
11-x=8
\]
\[
x=3
\]
3