1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Another number theory problem

  1. Mar 15, 2007 #1
    this is question found as part of a 25question 1hour long maths problem quiz i took today. It was just about the only one i couldn't do in the time, and even with another look after I can't do it :S I think it may involve maths I havnt come across or at least methodology i havn't (I 16 and only just started looking beyond class studies)

    1. The problem statement, all variables and given/known data

    Let N be the smallest integer such that 10 x N is a perfect square and 6 x N is a perfect cube.
    How many positive factors does N have?


    A 30 B 40 C 54 D 72 E 96

    2. Relevant equations

    3. The attempt at a solution

    right, Ill talk you through my attempt so far...


    I first of all tried a few values for N (the basics as tests like 0,1,2,3,5,10, etc..) and came to the logical conclusion that it wasn't going to be as simple as that, and probably would be a very large number (probably shoulda concluded that from the options but o well

    o yeh, I found out that 0 kinda works, but I'm guessing from the options that 0 doesn't count because 0 isn't a cube or a square? am I right?

    Then I tried using modular arithematic (I still a super noobie with it)

    N = 0 (mod 10) is a perfect square, while N = 0 (mod 6) is a perfect cube.

    From that I started working my way through the first half, trying lots of options for what makes the first bit work. I came up with stuff like N = 10, 40, 90, etc.. Not in a very logical approach but hey, mighta saved me time, but it didnt...

    I can't figure out how to find N?!?!?

    Have I approached the question all wrong though? is finding N entirely necessary or can you derive the factors somehow???!?!?

    I tried the later approach, looking for similarities between the factors of squares and cubes but I dunno if it was me being slow, or wierd, or whatever, (or oblvivious to the obvious) but I couldn't find anything that way...

    any help please :D

    and as per usualy, one step at a time ;)

  2. jcsd
  3. Mar 16, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    Clearly N has a prime factorization of the form 2^i*3^j*5^k. What constraints do your premises put on i,j,k?
  4. Mar 16, 2007 #3
    :S whats prime factorization?

  5. Mar 16, 2007 #4
    you can play some tricks involving divisibility.

    for instance, you have 10*N is a perfect square. so
    so 2 divides both sides, hence 2 divides a^2, so 2 divides a. (if 2 doesn't divide a, then 2 can't divide a^2 because 2 is a prime)

    so you have:


    cancel out the two

    rinse and repeat for 5. and similarly for 6N=b^3
  6. Mar 16, 2007 #5
    Sorry to be off topic but was that test the AMC?
  7. Mar 16, 2007 #6
    If [tex]10N = a^2[/tex], then [tex]2^{x},5^{y}, k_{1}^{z_{1}}, k_{2}^{z_{2}}... | N[/tex] where x, y can be expressed as 2*q - 1, where q is any positive integer. k refers to any prime, and z an even number.

    For [tex]6N = b^{3}[/tex], we have [tex]2^{a},3^{b}, l_{1}^{c_{1}}... | N[/tex] where a, b can be expressed as 3*q - 1, where q is any positive integer. l refers to any prime and c is a multiple of 3.

    Now we have concluded that 2, 3 and 5 must be factors of N. Also, we know what the factors of the respective exponents must be. Can you go on from there?
    Last edited: Mar 16, 2007
  8. Mar 17, 2007 #7
    it was the second round of the UKMT Maths Challenge (from the UK), and is called something like "the european pink kangaroo round", haha, funny name, lol

    anyways, yeh i reks i didnt through to the olympiad cause i working out i probs only got like 90 outa 135 or whatever it is...

    whats the AMC?
  9. Mar 17, 2007 #8
    o yeh, thnx for the help, ill try and go from there and get back to ya if i get stuck...
  10. Mar 17, 2007 #9


    User Avatar
    Science Advisor
    Homework Helper

    If you are going to keep looking at number theory problem it would be good to get to know about prime factorization. It's not even hard.
  11. Mar 20, 2007 #10
    o rite i know what prime factorization is lol

    just never heard it called that before cause at school i just get told it when you split a number into prime factors hehe, stupid me

    right i've decided i have no idea whats going on lol

    unfortunately, i not sure what any of your explainations mean or where to go from there.

    thnx for the help anyways, ill come to this problem once i read some number theory
  12. Mar 20, 2007 #11


    User Avatar
    Science Advisor
    Homework Helper

    So then do you agree N=2^i*3^j*5^k for some i,j,k? You can do it now.
  13. Mar 21, 2007 #12


    User Avatar
    Staff Emeritus
    Science Advisor

    In order that 10*N be a square, since 10= 2*5, N must have factors of 2 and 5. In order that 6*N be a cube, since 6= 2*3, N must have factors of 22 and 32. Since N is the smallest such number, it must be 22*32*5= 180. Now, I'll leave the hard part to you: how many distinct factors does 180 have?
  14. Mar 21, 2007 #13


    User Avatar
    Science Advisor
    Homework Helper

    Sorry, but the number of factors of 180 won't do him any good, since 10*180 is not a perfect square and 6*180 is not a perfect cube.
  15. Mar 21, 2007 #14
    Halls, something you've let something slip there...

    By the way Trailer Builder, the answer is ugly.
  16. Mar 21, 2007 #15


    User Avatar
    Science Advisor
    Homework Helper

    What's ugly about 2^5*3^2*5^3=36000?
  17. Mar 21, 2007 #16


    User Avatar
    Science Advisor
    Homework Helper

    Considering the prime factorization, counting factors is really easy as well... Trail Builder gave up too fast.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Another number theory problem
  1. Number theory problem (Replies: 1)

  2. Number theory problem (Replies: 9)

  3. Number theory problem (Replies: 11)