# Thread: probability of picking a ball (this is easy :P)

1. ## probability of picking a ball (this is easy :P)

if there are n balls in a bag, 1 black and n-1 white, and i take k balls out of the bag, how come the probability of taking a black is k/n?

I get that it works if you try adding the probabilities of getting black on the 1st, 2nd..kth attempt, but whats the logic behind it?

2. Just do the caculation: $\displaystyle{ \frac{\binom{1}{1}\binom{n-1}{k-1}}{\binom{n}{k}}}=?$

3. thanks, why is that the calculation?

4. Originally Posted by Aquafina
thanks, why is that the calculation?
Well you choose one black ball from 1 and k-1 white balls from n-1 white balls.