# Thread: Need help with the Mean formula

1. ## Need help with the Mean formula

I thought I understood the formula, but I can't figure out how to set it up for this problem. compute.jpg

2. Originally Posted by Inferno
I thought I understood the formula, but I can't figure out how to set it up for this problem.
Which formula? What did you plug in, where? Where did you get stuck?

3. this formula http://sph.bu.edu/otlt/MPH-Modules/B...eanFormula.png

. . . . .$\bar{x} \, =\, \dfrac{\Sigma X}{n}$

4. Originally Posted by Inferno
So now tell us what did you plug in? what did you get?

5. I am pretty sure I plugged the stuff in wrong. here's what I did for Set A

E20/6 = 5x10
E3.3 = 50
E=15.1

and set B

E20/6 = 50x10
E3.3=500
E=151.5

6. Originally Posted by Inferno
I am pretty sure I plugged the stuff in wrong. here's what I did for Set A

E20/6 = 5x10
E3.3 = 50
E=15.1

and set B

E20/6 = 50x10
E3.3=500
E=151.5
Okay that is not the way to work with mean (or average).

Tell us the definition of mean (or average).

7. mean is when you add all the numbers up and divide by how many there are

8. 10+20/6 = Set A is 5?

and 500+20/6= Set B is 86.7 ? are those right? I did that without the formula because the formula confuses me.

9. Originally Posted by Inferno
10+20/6 = Set A is 5?

and 500+20/6= Set B is 86.7 ? are those right? I did that without the formula because the formula confuses me.
The formula is not confusing. The problem is confusingly worded because it changes the definitions of the sets.

For set $A = \{x_1,\ x_2,\ x_3,\ x_4,\ x_5\}.$

$\displaystyle \bar x_A = \dfrac{\displaystyle \sum_{i=1}^5x_i}{5} = 10 \implies\sum_{i=1}^5x_i = 5 * 10 = 50.$ Follow that?

Now let's define a new set $H = \{x_1,\ x_2,\ x_3,\ x_4,\ x_5\, x_6\},\ where\ x_6 = 20.$

So $\displaystyle \bar x_H = \dfrac{\displaystyle \sum_{i=1}^6x_i}{6}.$ Still using the formula.

But what is the numerator in that formula equal to?

Here is the trick

$\displaystyle \sum_{i=1}^6x_i = \left(\sum_{i=1}^5x_i\right) + x_6.$ Does that make sense?

And we know what the two terms on the right of the equation equal.

$\displaystyle \sum_{i=1}^6x_i = 50 + 20 = 70 \implies \bar x_H = \dfrac{70}{6} \approx 11.67$

Now try the second problem on your own, and let us know what you get.

10. 50x10= 500

500+20= 520

520/6 = 86.7

still wrong? lol

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