# How to explain CWP without algebra?

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My oldest is working through CWP 3.    There was a problem there that went something like this:

A is 18 yrs older than B.  How old B will be when A is 3 times older than him?

The only way I know how to solve it is x+18=3x.   But I have no idea how to explain it otherwise.

Help!!

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For an 8 y/o? By playing with numbers and trying out!

A is 18 years older than B.

Let's think about when A is exactly twice as old as B.... try, try, try....

When B is 18, A is twice as old as B.

Oh, that's when the difference in their ages is the same as how old B is. Interesting.

Let's try and figure out how old B has to be so A is 3 times as old.

It has to be less than 18, because the older B gets, the less the 18 year difference matters. (If they are 72 and 90, for example)

play around some more...

So, maybe if B's age is only half the difference, that might work?

Let's try 9. When B is 9, A is 9+18=27. that's it! B is 9.

Edited by regentrude
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A is 18 yrs older than B.  How old B will be when A is 3 times older than him?

This bar chart shows that A is three times as old as B.

A: [         ][         ][          ]

B: [          ]

We also know that A is 18 years older than B, so...

[         18        ]

A: [         ][         ][          ]

B: [          ]

So if 2/3 of A's age is 18, then 1/3 = 9 which is how old B is.

Wendy

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My oldest is working through CWP 3.    There was a problem there that went something like this:

A is 18 yrs older than B.  How old B will be when A is 3 times older than him?

The only way I know how to solve it is x+18=3x.   But I have no idea how to explain it otherwise.

Help!!

CWP is typically used as a supplement to Singapore Math series and assumes that the student is familiar with the bar method as Wendy illustrated in her post.

The Instructor Manuals that accompany the Singapore Math series explains the bar method if you are unfamiliar with it.

Good luck.

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Huh....we ARE using SM.  I must have really screwed up bc I didn't teach him "bar method" at all!!!

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Huh....we ARE using SM.  I must have really screwed up bc I didn't teach him "bar method" at all!!!

The bar method doesn't show up in the regular books straight away if I recall correctly. I think some people use the CWP books a year later - ie. Use CWP 3 when doing SM 4  - that's what we did. I think by then the bar method had showed up in the regular books.

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The bar method doesn't show up in the regular books straight away if I recall correctly. I think some people use the CWP books a year later - ie. Use CWP 3 when doing SM 4  - that's what we did. I think by then the bar method had showed up in the regular books.

Yes, that's what we are doing.  We are finishing up 3B and I started him on CWP.  This problem was in the first 10, I think.

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Yes, that's what we are doing.  We are finishing up 3B and I started him on CWP.  This problem was in the first 10, I think.

The bar method is really well taught in Singapore's Process Skills books.

There is a thread going on right now in which a poster describes in detail what is covered in each of the Process Skills levels.

Wendy

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