# Help with MUS Rule of Four

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We jumped into MUS Pre-Algebra this year to give dd a year to solidify before going into algebra. We just encountered the "rule of four", which is not explained in the pre-algebra lessons (evidently it was introduced in a previous level).

Can someone explain to me what this is and how it works?

Thanks!

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deleted. not pertinent to question

Edited by regentrude
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It's used in dividing fractions to make the denominators the same.

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The "rule of four" is introducted in Epsilon (Fractions). It is used to compare, add, subtract and divide fractions. You multiply the denominator of the first fraction by the denominator of the second fraction, and then by the numerator of the second fraction, and then multiply the denominator of the second fraction to the denominator of the first fraction and the numerator of the first fraction. It is a "shortcut" to getting common denominators.

EX:

2/3 + 1/5 =

3 x 5= 15, 3 x 1= 3, so... 3/15, then

5 x 3= 15, 5 x 2= 10, so 10/15 becomes...

10/15 + 3/15=

Clear as mud? It seemed to me like going around your thumb to get to your elbow, but it works. It will not give you the lowest common denominator, but it will give you a common denominator.

Hope that helps.

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Weird thing is this really stuck with my ds and he uses is. It boggles me, but in the case when a CD is unclear, as the pp said, it's a quick way to get a common denominator, though not necessarily the lowest one.

We are in Zeta now, and he has trouble with some of the conventional shortcuts, but he remembers the rule of four.

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Okay, I get it now, I think. But I guess I don't understand why this is helpful. I think I need to see an explanation of what it's trying to accomplish. That may be the problem--since we didn't go through that level, we didn't get a video explanation (and those have been really good for dd in the pre-algebra program).

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Okay, I get it now, I think. But I guess I don't understand why this is helpful. I think I need to see an explanation of what it's trying to accomplish. That may be the problem--since we didn't go through that level, we didn't get a video explanation (and those have been really good for dd in the pre-algebra program).

The rule of 4 is explained with MUS fractional overlays and used to demonstrate how to add, subtract, and divide fractions. The approach used with division is novel to me, and DS loves it.

Yes, the videos would help you. If you google the "butterfly multiplication method" you'll get a close idea of the rule of 4.

Edited by Heathermomster
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Weird thing is this really stuck with my ds and he uses is. It boggles me, but in the case when a CD is unclear, as the pp said, it's a quick way to get a common denominator, though not necessarily the lowest one. ...

That is what it's about -- getting a common denominator.

I think the math intuition is firmer if you teach them to simply multiply each fraction by the other fraction's denominator/denominator -- in the case above, multiply the 2/3 by 5/5 and the 1/5 by 3/3. It does the same thing and is transparent to understand in terms of why you have the same values you started with, if the child understands that 5/5 = 1.

But we are math intuition geeks here; also the rule of four did NOT go over well. I am so glad to know that it works extremely well for some children! will help me in rec'ing math programs ...

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• 3 weeks later...

But we are math intuition geeks here; also the rule of four did NOT go over well. I am so glad to know that it works extremely well for some children! will help me in rec'ing math programs ...

It was a huge help for my math challenged son. It helped me too. (Our household has a distinct lack of math intuition.)

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I think the math intuition is firmer if you teach them to simply multiply each fraction by the other fraction's denominator/denominator -- in the case above, multiply the 2/3 by 5/5 and the 1/5 by 3/3. It does the same thing and is transparent to understand in terms of why you have the same values you started with, if the child understands that 5/5 = 1.

That is exactly what the rule of 4 does teach. The four is just the four numbers multiplied (numerator/denominator of fraction 1 then numerator/denominator of fraction 2). If MUS is being used they should understand that 5/5 = 1. Steve even draws a big outline 1 around fractions like 5/5 to remind kids it is equal to one.

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That is exactly what the rule of 4 does teach. The four is just the four numbers multiplied (numerator/denominator of fraction 1 then numerator/denominator of fraction 2). If MUS is being used they should understand that 5/5 = 1. Steve even draws a big outline 1 around fractions like 5/5 to remind kids it is equal to one.

I agree, that is mathematically equivalent to the rule of 4. When we went through this last year, doing it via the "rule of four" method vs. the multiplying each by 1 method made it much, much more obscure to Button and I didn't think the technique lent itself to facile work with humbers & strong, transparent "intuition" for what is being done. Certainly he never would pick the rule of four method over the multiply by x/x and y/y method: but clearly, others would!

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