Fifth Grade Homework - Nine digit pandigital prime number

[dblume]

Yesterday my daughter in the fifth grade got the following homework assignment "arrange the digits one through nine into a nine-digit prime number." (Note, since zero wasn't included, it's not really a pandigital number

Comments

[pastilla]

Did the teacher introduce a method or "trick" to the students beforehand . . . i.e. were the students exposed to any ideas that might encourage them to add all the digits up to see if they are divisible by anything?

The last two paragraphs explain (well, at least summarize) why you can't get a prime number out of these digits, and teaches a method.

My guess is that teaching the shortcut first will provide a foundation for understanding more complex factoring later.

If the teacher didn't teach this beforehand, and was trying to make a point through exhausting the kids . . . weak pedagogy, IMO!

[sjonsvenson]

teaching that sometimes no solution, no answer axists.

[tpederson]

yeah to teach to look for no solution...first and simplest rule I thought of was if digits add up to a multiple of 3 then number is divisible by 3.

45 is divisible by 3

[dblume]

You remembered that rule? I didn't. And even if I did, I wouldn't have felt comfortable telling the kids to use a rule that we hadn't proven yet. (Although by now, I know the proof.)