Probability

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rochy81: Newbie | Next Rank: 10 Posts; Posts: 3; Joined: Mon Oct 15, 2007 6:16 pm

Probability

by rochy81 » Tue Apr 29, 2008 8:07 pm

Can someone help solve this?

A hand purse contains 6 nickels, 5 pennies and 4 dimes. What is the probability of picking a coin other than a nickel twice in a row if the first coin picked is not put back?

Choices are:
8/25
12/35
13/35
9/25
17/25

And can you please tell me what level difficulty this would be considered because if this is considered easy... I might be in trouble..

Thanks!

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Source: Beat The GMAT — Problem Solving | Tue Apr 29, 2008 8:07 pm

luvaduva: Senior | Next Rank: 100 Posts; Posts: 84; Joined: Tue Feb 05, 2008 8:52 pm; Thanked: 8 times

by luvaduva » Tue Apr 29, 2008 8:50 pm

15 coins, 6 nickels, 9 not nickels

Prob of first pick not a nickel = 9/15
Prob of second not being a nick = 8/14

(9/15)*(8/14) = 72/210 = 24/70 = 12/35

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beeparoo: Master | Next Rank: 500 Posts; Posts: 200; Joined: Sun Jun 17, 2007 10:46 am; Location: Canada; Thanked: 9 times

Does anyone else have this problem too??

by beeparoo » Fri Jun 13, 2008 9:23 am

I am having difficulty with the possible AMBIGUITY of this Probability problem...

My interpretation of the Q, when it asks, "What is the probability of picking a coin other than a nickel twice in a row" is to also consider the possibility of picking a nickel ONCE during the two picks.

In summary, the picks could be:

(non-nickel, non-nickel)
(non-nickel, nickel)
(nickel, non-nickel)

But NOT (nickel, nickel)

I always have the most difficulty understanding what the question is asking for Probability-type questions. ARGH!!

Does anyone else have this problem too??

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g_beatthegmat: Master | Next Rank: 500 Posts; Posts: 100; Joined: Fri Jul 27, 2007 12:36 pm; Thanked: 6 times

by g_beatthegmat » Sun Jun 15, 2008 10:43 pm

GMAT will never come up with something ambiguous. Best approach to these questions is not to think that they are trying to confuse you

A hand purse contains 6 nickels, 5 pennies and 4 dimes. What is the probability of picking a coin other than a nickel twice in a row if the first coin picked is not put back?

This can be rephrased as -

A hand purse contains 6 nickels, 5 pennies and 4 dimes. What is the probability of picking a non-nickel coin twice in a row if the first coin picked is not put back?