Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Fundamental Counting Principle problem

  1. Dec 15, 2003 #1
    The dial on a 3 number combination lock contains markings to represent the numbers from 0 to 59. How many combinations are possible if the first and second numbers differ by 3?

    What I did was:

    1st number: It can be any of the 60 numbers (if we take 0 also as a #)
    2nd number: I think since there are two possibilities, either 3 greater than 1st # or 3 less
    3rd number: Since you've already take a number for the first one, and you must choose either of 3 less or 3 greater than the first number as the 2nd #, you must in the end have 57 #'s left to choose from

    Therefore the answer I think is 60 x 2 x 57

    Is that right?
     
  2. jcsd
  3. Dec 15, 2003 #2

    HallsofIvy

    User Avatar
    Science Advisor

    Not quite. If the first number is 57, 58 or 59, then the next number CAN'T be "3 larger" but only 3 less. If the first number is 0, 1, or 2, the next number CAN'T be "3 less but only 3 more.
    That is, if the first number is 3 to 56 (54 numbers) then there are 2 possible second number but if the first number is 0, 1, 2, 57, 58, or 59, then there is only 1 possible second number. There are 54*2+ 3+ 3= 114 possible two digit combinations for the first two numbers. There are 114*60= 6840 such three digit combinations.
     
  4. Dec 15, 2003 #3

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Halls,

    While he didn't explicity state it, I would think that you can go "over the top" and consider 59 and 2 to differ by 3.

    Pi,

    I think you're almost correct. However, if you choose one of 60 numbers, then choose one of the 59 remaining, there are 58 left -- NOT 57.

    - Warren
     
  5. Dec 15, 2003 #4
    Thanks for the replies guys. Hall I dont understand why you did 54*2 and added 6
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook