# Homework Help: Calulating the Mean

1. Jan 25, 2010

### Dollydaggerxo

1. The problem statement, all variables and given/known data
I realise this is probably an obvious question, but if there are two groups, say group A and group B, where group A was full of old people, and group B full of children.
The mean age was 67 for group A
and the mean age was 5 for group B.
If these two groups joined together to make one big group, what is the new mean?

3. The attempt at a solution

is it just ( 67 + 5 ) / 2
so it would be 36?

2. Jan 25, 2010

### dacruick

yes assuming groups A and B are the same size.

3. Jan 25, 2010

### Dollydaggerxo

Oh i see, so what happens if they are different sizes?

say A is 4 ad B is 5.

4. Jan 25, 2010

### dacruick

well then that is what is called different weighting. For example, take your grades. if you had two tests both worth 50% of your mark. They are weighted the same. If you get 70% on one and 80% on the other, your average will be (70+80)/2.

however if one is worth 25% and one is worth 75%, and you got the same marks respectively, your final mark would be 0.25(70) + 0.75(80)

5. Jan 25, 2010

### dacruick

and if you dont see the mathematic pattern; (70+80)/2 is exactly the same as 0.5(70)+0.5(80)

6. Jan 25, 2010

### Dollydaggerxo

ah brilliant, I never thought of it like that!
Thankyou

7. Jan 25, 2010

### dacruick

Yes, everything is simple once you understand it. This same concept is applied on the periodic table. I don't know how well you know chemistry, but you must know that each different element has an atomic weight. You also might know of isotopes. Carbon is the atomic number 12 on the periodic table which means that it has 12 protons. But there are also isotopes of carbon such as carbon 13(which has 13 protons). So from the samples scientists see that 3% of the chunk of carbon is carbon 13, and 90% is carbon 12, and 7% is carbon 11. Then the weight that shows up on your periodic table is as follows:

0.03(weight of carbon 13) + 0.90(weight of carbon 12) + 0.07(weight of carbon 11)

8. Apr 15, 2010

### Dickfore

All isotopes of carbon have 6 protons. You clearly do not understand the periodic table.

9. Apr 16, 2010

### dacruick

haha i clearly meant 13 protons and neutrons. but who cares this is a physics forum. tehre was no issue insofar as the concept explained. what are you being a dickfor?

10. Apr 17, 2010

### jeppetrost

Is it only me, who saw the kinda joke..? "What are you being a dick for, Dickfore?"

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