Obtaining sample variance from grouped data for goodness of fit test

#1
This is a practice question I came across when dong some goodness of fit test examples.A company sells cloths by mail order.The size of clothes is defined by hip size; thus the height of customers of a particular customer may vary considerably.
Data set of heights sent in by customers of size 18 is given.Data set includes class interval,class mid mark and frequency.

a) Given that ∑f=265, ∑fx=40735, ∑fx^2=6299425 estimate mean and standard deviation of heights.
b)The company decides that it is not possible to produce a range of garments for a particular size suitable for customers of different heights.A single height must be chosen and it is proposed that mean height should be chosen for this.Comment on this suggestion as it applies to customers of size 18 and make an alternative proposal.

My question is how can I calculate sample variance from ∑f=265, ∑fx=40735, ∑fx^2=6299425
From V(X)=E(X2)- E[X]2 what I get is population variance and not sample variance right? For sample variance I should divide by n-1.So how can I get sample variance value?

Please help me to answer part b?