# Probability question

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

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Apr 7, 2007
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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

Thank you!

3. ### harrygt

Joined:
Jun 16, 2008
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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]