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sali768

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Eight girls and 12 boys were asked how much money they had in their wallets. The eight girls had an average of $8.12. The 12 boys had an average of $6.24. What was the average amount that all 20 people had in their wallets?

This is how I did the question.
[8(8.12) + 12(6.24)] / 20 = 6.99

Please let me now if I did it correctly or not.
Thanks in advance
 
Yes, that is correct. But the real question is do you understand why that is the way to calculate this?

The average of n numbers is their sum divided by n. If the average of the eight girls' money was $8.12 then the total amount or money they had was 8(8.12)= $64.96. If the average of the 12 boys' money was $6.24 then the total money they had was 12(6.24)= $74.88.

So, altogether, the 20 people had a total of 8(8.12)+ 12(6.24)= 64.96+ 74.88= $139.84 so the average was (8(8.12)+ 12(6.24))/20= 139.84/20= $6.99 as you say.
 
This was the question given to parents during open house by my daughter's math teacher. She marked my answer wrong. I was surprised! I've put my question on this forum to confirm if I were correct.
Now I don't know if I should let her know or should just ignore?
 
According to the teacher the correct answer is $7.18.
How could that be possible? This class is Advance Algebra 2 and she's been teaching for many years.
 
sali768 said:
According to the teacher the correct answer is $7.18.
How could that be possible? This class is Advance Algebra 2 and she's been teaching for many years.
Click to expand...

She is giving the mean of 6.24 and 8.12, and her answer would be correct if the number of boys and girls was the same. But they are not, and as others have stated, your use of a weighted average was correct. I would let her know if it were me, just so she doesn't cause undue confusion.
 
The teacher taking the average of the two amounts does not surprise me at all. This is so depressing to see (all the time!)
Please ask your daughter to join this forum so we can clear everything up that .....
 
Please get in touch with the teacher or her head of department (in a constructive and kind way). There are many teachers out there teaching maths when they shouldn't be. Some are forced to teach maths because there is a shortage of maths teachers. They are probably great History, PE, or anything else teachers but maths .. no! It shouldn't happen but that is the reality.
 
Over the years my worst students have been math ed majors--future high school math teacher. Not one of them ever started in Calculus but rather in arithmetic or algebra. How do you graduate high school thinking that you will be a high school math teacher given that you did not even learn algebra (or even worse arithmetic!) in high school? Unfortunately the answer is easily, as they appently let anyone be a high school math teacher.

Many years ago I taught FT at a community college in NYC where you needed a PhD in math or math ed to be hired (the PhD requirement came after I was hired and for the record I have a Masters degree in math). Many many FT math faculty with PhDs in math education ONLY taught arithmetic for their full time load. I enjoy teaching arithmetic, but 5 sections of arithmetic each semester for your whole career is crazy.
 
Harry_the_cat said:
Please get in touch with the teacher or her head of department (in a constructive and kind way). There are many teachers out there teaching maths when they shouldn't be. Some are forced to teach maths because there is a shortage of maths teachers. They are probably great History, PE, or anything else teachers but maths .. no! It shouldn't happen but that is the reality.
Click to expand...
The head of the some departments are not much better.
I have a friend who has a PhD in math and in grade school asked his teacher how the area of a circle was (for example) 5 sq inches. The teacher responded well you can only put in 5 squares (with no units!!). When my friend said that if you make the square smaller you will be able to put more than 5 squares. The teacher said to go ask the chairperson. He did and did not get an answer from her either. This is not complex stuff but they do not get it. I was tutoring a math teacher and she told me that her boss knew that she could not understand the difference between area and perimeter. Again, unacceptable. But it is what itis.
Sorry for going on about this but this topic really gets to me
 
Harry_the_cat said:
There are many teachers out there teaching maths when they shouldn't be. Some are forced to teach maths because there is a shortage of maths teachers. They are probably great History, PE, or anything else teachers but maths .. no! It shouldn't happen but that is the reality.
Click to expand...
Don't think the math teachers who shouldn't be there do not have degrees in math education.

If you have a masters degree in math or a PhD in math you are NOT qualified to teach math in the public schools because you did not take education courses. How sad!!
 
It could be that this teacher simply made an error, perhaps in haste, and is otherwise competent. :)
 
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