Permutations and Combinations...

Arnold has ten 1000-pesos bills of which three are counterfeit. If he selects four of them at random, what is the probability of getting two good bills and two counterfeit bills?

Question

Permutations and Combinations...

Arnold has ten 1000-pesos bills of which three are counterfeit. If he selects four of them at random, what is the probability of getting two good bills and two counterfeit bills?

chaise · Answer

I had a maths test with this sort of stuff on it today.

There are 7 REAL BILLS
There are 3 FAKE bills
There are 10 bills to choose from
There are 4 BILLS being chosen altogether

(7C2 x 3C2)/10C4
= 3/10 or 0.3

I hope this is right. Verification with Polpak might be nice :)

chaise · Answer

Do you need me to explain this further?

chaise · Answer

I don't know the method that Polpak used - Got similar answers though. :)

anonymous · Answer

Bah. We got the same answers. You just use the nCr notation. Which is the same as $\large n \choose r$

anonymous · Answer

And I screwed up the percentage at the end for some inexplicable reason.

chaise · Answer

We all make mistakes - Are you able to help me with another question please Polpak?

anonymous · Answer

Certainly

hero · Answer

Does pesos still exist?

anonymous · Answer

yup, they do.