Need help from the CP math whizzes

[cdcox]

Can the kids factor 2007 into 3x3x223 quickly?

[Frosty]

Originally Posted by cdcox

Can the kids factor 2007 into 3x3x223 quickly?

This is 7th grade level, so they should be able to, once they figure out that 223 is prime.

[cdcox]

Originally Posted by Frosty

This is 7th grade level, so they should be able to, once they figure out that 223 is prime.

Yeah, the factoring of 2007 is the hard part. If they guess it is divisible by 3, you get 669, which is clearly divisible by 3. Then they just have to go with their gut that 223 is prime.

It is pretty easy to see that 223 isn't divisible by 2, 3, 5, 7, or 11, so guessing that it is prime is pretty safe at that point.

[patteeu]

Originally Posted by cdcox

Yeah, the factoring of 2007 is the hard part. If they guess it is divisible by 3, you get 669, which is clearly divisible by 3. Then they just have to go with their gut that 223 is prime.

It is pretty easy to see that 223 isn't divisible by 2, 3, 5, 7, or 11, so guessing that it is prime is pretty safe at that point.

They should be able to recognize that it's divisible by 3 if they know the simple "trick" that any number whose digits add up to a number divisible by 3 is, itself, divisible by 3. I'm sure you and frosty are well aware of this, but just in case, I thought I'd mention it.

2+0+0+7=9

9 is divisible by 3, therefore 2007 is divisible by 3.

[aturnis]

Originally Posted by patteeu

They should be able to recognize that it's divisible by 3 if they know the simple "trick" that any number whose digits add up to a number divisible by 3 is, itself, divisible by 3. I'm sure you and frosty are well aware of this, but just in case, I thought I'd mention it.

2+0+0+7=9

9 is divisible by 3, therefore 2007 is divisible by 3.

Never seen this trick before. Why aren't we taught all the tricks in school? Might help.