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 Post subject: maths question - help please!Posted: Wed Dec 10, 2008 6:42 pm

Joined: Wed Jan 09, 2008 1:21 am
Posts: 179
DS1 is struggling with a maths question and I'm afraid I feel lost as well. He has worked it out by trial and error but there must be a sensible way of doing this.

no of boxes.......frequency

2........................ 5
3........................ 12
4........................ x

if mean number of boxes is 3.6 what is value of x?

DS has worked out the answer to be 38 but as I say by trial and error. is there a proper way of doing this?

(sorry about the dots; I couldn't work out how to get them in columns on here otherwise)

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 Post subject: Posted: Wed Dec 10, 2008 7:39 pm

Joined: Mon Feb 12, 2007 1:21 pm
Posts: 14003
mean =[ (2x5) + (3x12) + (4 multiplied by x)] /(5+ 12 + x)

3.6(17 + x) = 10 + 36 + 4x

etc

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 Post subject: Posted: Wed Dec 10, 2008 7:54 pm

Joined: Sat Nov 24, 2007 11:09 pm
Posts: 218
I work out 38 as well

mean =[ (2x5) + (3x12) + 4X] / (5+12+X)

3.6 = [10 +36+ 4X] / 17+X

3.6 (17+X) = 46+ 4X

61.2 + 3.6X = 46 + 4X

61.2 - 46 = 4X -3.6X

15.2 = 0.4 X

15.2 / 0.4 =X

38 = X

I hope it makes sense.

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 Post subject: Posted: Wed Dec 10, 2008 9:19 pm

Joined: Wed Jan 09, 2008 1:21 am
Posts: 179
thank you essex mum and guest55
I could get the first line - i.e. the equation but I always struggle with remembering how to simplify / move the equation around.

When I see any equation this big fog comes down and I can see the same thing happening for my son. I can never get my head around when you can move numbers from one side to another. I am going to print out your solutions and see if we can work through them together.

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 Post subject: Re: maths question - help please!Posted: Wed Dec 10, 2008 11:19 pm

Joined: Fri Jul 06, 2007 8:31 pm
Posts: 1245
Greta2 wrote:
2........................ 5
3........................ 12
4........................ x

2*5+12*3+4x = (5+12+x)*3.6
46+4x = 61.2 + 3.6x
0.4x = 15.2
x=38

But... guessing isn't a bad approach...

Guess 40...

and then go down a bit?

Regards
SVE

_________________
Animis opibusque parati

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 Post subject: Posted: Thu Dec 11, 2008 6:55 am

Joined: Mon Feb 12, 2007 1:21 pm
Posts: 14003
Not sure about guessing ... how would you know what is a good number to start with? We know it has to be bigger than 17 to 'pull' the mean up to 3.6 but after that I think it would take too long.

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 Post subject: Posted: Thu Dec 11, 2008 8:20 am

Joined: Wed Jan 09, 2008 1:21 am
Posts: 179
Guessing took a long time.... as he started at 1!

So yes he got the answer after 5 pages of trying but with no real idea of what he was doing - certainly didn't twig the number would have to be more than 17.

I have now gone over the solutions given myself and will go through this with him over the weekend. It is my own lack of confidence about moving equations around that is rather poor - it makes me wish I understood it better. DS1 does struggle with this type of thing and like me he needs very concrete methods which go slowly (only one change per line type of method).

I sort of hope his teacher may realise he doesn't understand - perhaps the lack of working out may give the teacher a clue!

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 Post subject: Posted: Thu Dec 11, 2008 9:01 am

Joined: Sat Nov 24, 2007 11:09 pm
Posts: 218
Hi, Greta 2

When I learnt this some years ago, my teacher always reminded us using a *SWING & CHANGE METHOD*, i.e looking at the signs (+,-, x, divide ) before the number. In order to make the equation balance, change the sign in an opposte direction on the other side ( + to -. - to +, x to divide, divide to x)

e.g

2y- 10 = y+2

2y- y = 2 + 10 ( y on the right becomes -y on the left, -10 on the left becomes +10 on the right in order to balance the equation)

y = 12

I teach my son using this method ( similar to his school method, may be a bit quicker ) and explain to him that if you get rid one y on the right side, you have also to get rid one y on the left side to balance the equation.

I hope this does not confuse you.

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 Post subject: Posted: Thu Dec 11, 2008 4:09 pm

Joined: Wed Jan 09, 2008 1:21 am
Posts: 179
Thank you so much essex mum - that did not confuse me. I like the idea of "swing and change" - that is starting to make some sense to me.

I am sure I must have known this once but it has made me realise just how rusty my maths is.

It also makes me wonder how I ever got my 'O' level! - I think I was the victim, so to speak, of some good teachers who managed to get me to know just about enough to get a reaonable O level grade but without ever really understanding what I was doing. That's what I want to avoid for my son! I would like him to be able to understand it as well as just be able to do it.

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 Post subject: Posted: Thu Dec 11, 2008 8:21 pm

Joined: Mon Feb 12, 2007 1:21 pm
Posts: 14003
"Swing and change' is a biit dodgy as when you divide you change the side but not the sign.

Thinking about a balance is OK - if we subtract 2x on the left we do the same on the right. If we divide the lfet we divide the right.

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