# Thread: Can any one explain this question to me, thxs.

1. ## Can any one explain this question to me, thxs.

- From a box containing 3 white balls and 7 black balls, 30 balls are drawn with replacement. Find the probability of getting 9 and 10 white balls?

I'm having a little bit of trouble finding how to work it out any help would be much appreciated.

2. Originally Posted by Funnel
- From a box containing 3 white balls and 7 black balls, 30 balls are drawn with replacement. Find the probability of getting 9 and 10 white balls?

I'm having a little bit of trouble finding how to work it out any help would be much appreciated.
Hi Funnel,

this is binomial.

As balls are being replaced, and there are 10 balls in total, the probabilities are always

$\displaystyle P(w)=0.3,\ P(b)=0.7$

Hence, you need to calculate the binomial term for 9 whites and 21 blacks,
and seperately for 10 whites and 20 blacks

$\displaystyle \binom{30}{9}0.3^90.7^{21}$ or $\displaystyle \binom{30}{21}0.7^{21}0.3^9$

$\displaystyle \binom{30}{10}0.3^{10}0.7^{20}$ or $\displaystyle \binom{30}{20}0.7^{20}0.3^{10}$