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Mean of a frequency table

This is the question: http://imgur.com/aSdFQxb

This is what I'm doing:

I work out the middle number of each weight and add them up
175+
225+
275+
325+
375= 1375
then i add up the frequencies
3+4+5+6+2=20
Then I divide 1375 by 20 and get 68.75.
The answer at the back of my book is 272.5?

What am I doing wrong?
Homework is due tomorrow

Reply 1

ghostwalker

Original post by LibertyGoose

What am I doing wrong?

Certainly you need to sum the frequencies, BUT you don't sum the mid-points, you need to multiply each mid-point by its corresponding frequency and then sum them.

Reply 2

LibertyGoose

Original post by ghostwalker

certainly you need to sum the frequencies, but you don't sum the mid-points, you need to multiply each mid-point by its corresponding frequency and then sum them.