Calculating the Mean of a frequency table.

This is the question:

​< link to objectionable page removed >

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

LibertyGoose said:

This is the question: <link removed>

Click to expand...

There are two exercises in the image. Which one did you mean? Perhaps the second one?

LibertyGoose said:

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.

Click to expand...

You divided the sum of ONE of each value by the sum of ALL the numbers of values. Try finding the total (being the sum of the values, each multiplied by the number of occurrences of its value) and then dividing by your second sum.

LibertyGoose said:

This is the question: <link removed>

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

Click to expand...

You did not multiply the middle value by the number of samples from that weight. Instead you should have
3(175)= 525
5(225)= 1125
4(275)= 1100
6(325)= 1950
2(375)= 750
which adds to 5450.

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?

Click to expand...

5450/20= 272.5

What am I doing wrong?
Homework is due tomorrow

Click to expand...