1. Dismiss Notice
  2. Guest, be sure to check out How To Get Into Dental School, our free downloadable PDF with step-by-step details for dental school applicants!
    Dismiss Notice
  3. Dismiss Notice
  4. Download free Tapatalk for iPhone or Tapatalk for Android for your phone and follow the SDN forums with push notifications.
    Dismiss Notice

Probability question

Discussion in 'DAT Discussions' started by Mstoothlady2012, Jun 20, 2008.

  1. Mstoothlady2012

    5+ Year Member

    Joined:
    Apr 7, 2007
    Messages:
    1,714
    Likes Received:
    4
    Status:
    Dental Student
    What am I doing wrong here?

    A bag contains 3 gold coins & 7 silver coins. The bag contains nothing else. If 3 coins are selected at random and without replacement from the bag, what is the probability that exactly 2 coins are silver?

    7/10 * 6/9 * 3/8 = 7/40 <--- But this is not the answer:eek:

    Thank you!
     
  2. Note: SDN Members do not see this ad.

  3. harrygt

    2+ Year Member

    Joined:
    Jun 16, 2008
    Messages:
    802
    Likes Received:
    5
    Status:
    Pre-Dental
    Hey buddy

    you gotta use combinations for this question, because the order is not important. The one you claculated in only one of the many ways that can happen, and finding the other ways, multyplying them, and adding all of them up at the end would take you five minutes at least.

    Using combination:

    C(7,2) * C(3,1) / C (10,3) = 21 * 3 / 120 = 21/40
    I hope you can find that in one of the choices. [* you gotta use the n! stuff to find the answer to each of the combinations. Let me know if you don't know how to calculate them]
     
  4. Mstoothlady2012

    5+ Year Member

    Joined:
    Apr 7, 2007
    Messages:
    1,714
    Likes Received:
    4
    Status:
    Dental Student
    oh ok I get it. Uh! These are so hard for me, just took a quiz and got 2/10:( Thanks!
     

Share This Page