[tex]\large\underline{\sf{Solution-}}[/tex]
We know,
Mode of the continuous series is given by
[tex]{\boxed{ \boxed{\sf{Mode = l + \bigg(\dfrac{f_1 - f_0}{2f_1 - f_0 - f_2} \bigg) \times h }}}} \\ \\ [/tex]
where,
l is lower limit of modal class.
[tex] \sf{f_1} [/tex] is frequency of modal class
[tex] \sf{f_0} [/tex] is frequency of class preceding modal class
[tex] \sf{f_2} [/tex] is frequency of class succeeding modal class
h is class height.
Now, from given data we concluded that
[tex]\rm \: f_0 = 6 \\ [/tex]
[tex]\rm \: f_1 = f \\ [/tex]
[tex]\rm \: f_2 = 8 \\ [/tex]
[tex]\rm \: l = 65 \\ [/tex]
[tex]\rm \: h = 15 \\ [/tex]
[tex]\rm \: Mode = 75 \\ [/tex]
So, on substituting the values in above formula, we get
[tex]\rm \: 75 = 65 + \bigg[\dfrac{f - 6}{2 \times f - 6 - 8} \bigg] \times 15\\ [/tex]
[tex]\sf \: 75 - 65 = \dfrac{f - 6}{2f - 14} \times 15 \\ \\ [/tex]
[tex]\sf \: 10 = \dfrac{f - 6}{2f - 14} \times 15 \\ \\ [/tex]
[tex]\sf \: 2 = \dfrac{f - 6}{2f - 14} \times 3 \\ \\ [/tex]
[tex]\sf \: 2(2f - 14) = 3(f - 6) \\ \\ [/tex]
[tex]\sf \: 4f - 28 = 3f -18 \\ \\ [/tex]
[tex]\sf \: 4f - 3f = 28 -18 \\ \\ [/tex]
[tex]\bf\implies \:f = 10 \\ \\ [/tex]
[tex]\rule{190pt}{2pt} \\ [/tex]
Additional Information :-
1. Mean using Direct Method
[tex]\boxed{ \rm{ \:Mean = \dfrac{ \sum f_i x_i}{ \sum f_i} \: }} \\ \\ [/tex]
2. Mean using Short Cut Method
[tex]\boxed{ \rm{ \:Mean =A + \dfrac{ \sum f_i d_i}{ \sum f_i} \: }} \\ \\ [/tex]
3. Mean using Step Deviation Method
[tex]\boxed{ \rm{ \:Mean =A + \dfrac{ \sum f_i u_i}{ \sum f_i} \times h \: }} \\ \\ [/tex]